Shellfish is a broad term for crustacean, mollusc and cephalopod seafood. Each has its own varieties and its own peculiarities of preparation. Most shellfish is easy to cook and is rich in minerals and bursting with flavour.

Crustaceans include crabs, langoustines, lobsters, prawns and shrimps. Crustaceans usually have segmented bodies and are often enclosed in a hard shell. Most have ten legs, sometimes including one pair of front claws or pincers.

Molluscs are invertebrates and include three main groups of seafood: gastropods, bivalves and cephalopods. Gastropods are single-shell molluscs and include the whelk and winkle. Bivalves are enclosed in two shells and include clams, cockles, mussels, oysters and scallops. Cephalopods are molluscs without a shell and include cuttlefish, octopus and squid.

Shellfish is available fresh and raw, frozen, smoked, dried, canned and even pickled.

Raw or improperly cooked seafood and shellfish can contain harmful viruses and bacteria. Bivalves such as clams and oysters are filter feeders, meaning that water (clean or polluted) passes through them. Toxins in algae ingested by shellfish can accumulate in their flesh and reach levels dangerous to humans. Pregnant women should only eat shellfish (including oysters) that have been thoroughly cooked.

Shellfish have seasons, and although cultivated mussels and oysters are available year-round, other varieties aren't so accommodating. Langoustines are best from late spring to late autumn. Weather conditions affect the availability and cost, as does the spawning season (when the flesh of shellfish softens and has less flavour). Some species migrate, so might only be available frozen in certain months.

As consumers, we're given little information about how and where our shellfish has been caught, landed, frozen and transported. If you're lucky enough to have good fishmongers in your high street or your local market, use them and ask about what you're buying.

When buying crustaceans they should smell fresh and sweet, of the sea (with no hint of ammonia); they should glisten with moisture; legs, heads, pincers and tails shouldn't be loose or detached from the body. Their flavour is best if bought alive and cooked when very fresh. Crabs and lobsters should feel heavy for their size. Raw prawns should be firm and springy with bright shells. Frozen prawns are available year-round but buy frozen raw prawns with their shells on if possible, avoiding the bland frozen cooked prawns.

When buying molluscs, look for those with shells that are smooth and shiny. Bivalves' (clams, cockles, mussels, oysters) shells should be tightly shut or should shut immediately if you tap them - scallops are the exception because they don't need to be alive on cooking). Avoid frozen bivalves as they usually taste rubbery when cooked. When buying mussels, colour is not indicative of quality - orange flesh tells you it's female while a whiter mussel usually suggests a male - though you may prefer rich, orange-coloured mussels to those with a paler hue.

Each region of the world has its favourite local crab. In Europe it's the Brown crab and the Spider crab; along the Eastern seaboard of the United States it's the Blue crab; and along the Pacific coast of the United States it's the delicious Dungeness crab. The soft brown flesh from under the hard upper shell of all crabs is rich, and contrasts well with the sweet, dense white flesh found in the claws and body. Males often have larger claws and more white flesh. However, females may have 'coral', a sumptuous red roe. All crabs must be sold either live or cooked; the meat is just as good eaten hot as it is cold. The following are the main types of crab available:

European Brown crab's available all year; it reaches 20-25cm/8-10in across and has heavy front claws with almost-black pincers, a rusty-red or brown shell, and red hairy legs mottled with white.

The Atlantic Blue crab has a blue-brown shell and grows to 20cm/8in, but is usually sold smaller. When newly 'moulted' (crabs regularly shed their hard shells), the crabs are caught with soft-shells and deep-fried for eating whole.

The Dungeness crab reaches up to 20cm/8in and has plenty of delicious white, dense sweet meat. The meat in the shell, notably different for its pale greyish-green colour, also has a great flavour.

Spider crabs are popular in France and Spain. This sweet-flavoured crab has no large claws and, indeed, resembles a big spider.

Mitch Tonks shows how to pick the meat from a crab in his video recipe for Dressed crab.

Langoustine/Dublin Bay Prawn is also known as Norway lobster or scampi. These orange-pink shellfish from the North-east Atlantic and Mediterranean resemble small, slim lobsters. They're delicious if freshly caught and cooked. The best specimens can be bought from late spring to late autumn and are cooked in the same way as lobster. Because of their smaller size, they need less cooking. Often only the tail is sold. They should be available frozen all year.

Lobster is a relative of the crab. When it's alive its shell is mottled with green, blue and black, but this turns the distinctive bright red on cooking. Lobsters can grow to be 4.5kg/10lb, but are best eaten at about 500g-1.4kg/1lb-3lb. Smaller lobsters are more tender. Cold-water lobsters from more northern seas are considered the finest. Males have denser, meatier flesh, but females have a more subtle flavour and an orange 'coral' roe - often used to colour sauces.

Prawns are divided into many species, varying in size from 5-18cm/2-7in long. King prawns grow even larger - up to 23cm/9in. Prawns also come in many colours, from the familiar common pink prawns to browny-blue tiger prawns. In the United States prawns are known as shrimp. There are two main types available in the UK: cold-water prawns, which come from cool northern waters, and warm-water 'tiger' prawns. If you can, avoid buying tiger prawns unless they are labelled organic. Non-organic tiger prawns are often farmed in large fish farms in some tropical countries, where farming methods have been the cause of degradation of coral reefs and mangrove swamps. Farmers who produce organic tiger prawns must follow requirements including mangrove replanting, limited stocking densities and strict health and feeding guidelines.

Langoustines/Dublin Bay prawns should be boiled in well-salted water for up to ten minutes if bought raw. Eat with melted butter or aioli. Pre-boiled langoustines should be reheated gently; don't re-cook or they'll toughen. Alternatively, eat them cold in a salad with mayonnaise or vinegar dressing. They're tasty grilled with oil and garlic, or deep-fried in batter.

Lobsters are almost entirely edible apart from the transparent bag-like stomach and the dark intestines, which should be removed with a sharp knife point. Keep the creamy green-grey liver (the 'tomalley'), which has excellent flavour. Obviously the shell is too hard to eat but should be reserved and used for fish stock or soup and as the base for a bisque or lobster sauce. Cooked lobster can be served hot or cold and is delicious with melted butter, lemon juice or mayonnaise. If you're feeling adventurous, try making lobster mousse or risotto or, to stretch it further, watch how Mitch Tonks cooks it with pasta in his video recipe for Spaghetti with lobster and basil.

Prawns can be boiled in salted water for a couple of minutes if bought raw and are also delicious grilled, deep-fried, stir-fried or barbecued. Make sure they're thoroughly cooked but take car not to overcook them or they'll become woolly and tough. If you've bought cooked prawns they should be reheated gently so as not to toughen the meat. Common pink prawns are traditionally eaten cold with lemon juice and brown bread and butter. Chinese cookery has some great recipes - sweet and sour prawns, for instance. Prawns also feature in the popular pad Thai noodle dish, and make a splendid curry or risotto.

Mitch Tonks shows how quick they are to cook in his video recipe for Chargrilled sweet and sour prawns.

Squid or calamari is available most of the year fresh or frozen, squid vary in size from small specimens of 5-7cm/2-3in, to larger ones of about 25cm/10in; few shellfish fare so well when frozen, and preparing squid is easy once you know how (save the silvery ink-sac from the innards to colour the dish later).

Octopus isn't as popular in the UK as elsewhere in the world, but when properly cooked it's delicious, meaty and tender. It's almost always sold already cleaned and only needs tenderising before cooking.

Squid must either be cooked very quickly or for very long, otherwise it will be tough. Cooked properly it becomes sweet and tender. Simply grill quickly or slice into rings, batter and deep-fry. Larger squid can be stewed with tomatoes, red wine and olive oil. Squid is delicious stuffed, and can be used in paella.

Octopus can be tough if not tenderised before cooking. The traditional way to tenderise the meat is to hurl it against rocks; an easier method is to simmer it for an hour. After pre-cooking, grill or stew the octopus until tender. Rice with octopus is a popular fried dish from Portugal, or try stewing it in Elizabeth David's Genoese fish stew or burrida.

Clams come in many shapes, from the tiny 'vongole' to the razor-shell clams shaped like cut-throat razors.

Cockles are rarely sold fresh; usually you'll find them preserved in vinegar or brine and served as appetisers; if you're lucky enough to find live fresh cockles, cook them as you would mussels.

Mussels are familiar as the small blue-black shells attached to rocks and piers, but the most common mussels available in British fishmongers and supermarkets today are the cultivated 'rope-grown' variety. These are less likely to be contaminated because they're grown far out at sea and are free from dirt and sand.

Oysters have several species around the globe; the best British oysters, called 'native' or 'flat', are quite hard to come by but are now farmed, particularly around the coasts of southern England (other British oysters include the rock oyster). Natives are available from September to April, though best from late October to late February when the sea is colder. Cultivated Pacific oysters are available all year.

Scallops are highly regarded and expensive too; the shells are rounded and fan-shaped, varying in size from the smaller queen scallops (7cm/3in across) to larger ones of about 18cm/8in across; the edible part is the round, white muscle and the orange and white roe, called 'coral' (the frilly gills and mantle should be used for soups and stocks).

Whelks are chewy, juicy and salty and resemble pointed snails. They're usually sold already cooked

Winkles are small black sea snails that make tasty appetisers. They're usually sold cooked.

Mussels need very little cooking. Place them in the bottom of a large, heavy-bottomed pan with a small amount of liquid and turn up the heat to steam them. As soon as the shells start gaping open, you know they're ready. Don't overcook them or you'll end up with rubbery flesh. Discard any that fail to open fully.

Mussels are delicious with a wide array of flavours. Steaming them in vermouth or white wine along with a spiking of shallots, some garlic, and a few herbs is traditional in some European countries, or try them with a combination of South-east Asian flavourings such as coconut, ginger, lemongrass and chilli. The cooking liquor is half the joy of eating mussels, so have plenty of crusty bread on stand-by for soaking it up.

Oysters should have their shells scrubbed well before opening. To open the shell (called shucking), use an oyster knife. Discard any oysters with an unusual or unpleasant smell. Native oysters are best eaten raw - as soon as possible after being caught. Cooked rock oysters are good for chowder or stews. They're also good served with a white wine sauce. Alternatively, grill with shallots and butter. Oysters wrapped in bacon and grilled are known as angels on horseback. Smoked oysters are delicious, though difficult to find.

Whelks and winkles are great steamed, stir-fried or coated in breadcrumbs and fried.

Try Rick Stein's whelk fritters

Cockles and clams can be grilled, sauteed or steamed in their shells, but the most famous clam dish is New England clam chowder. Clams are delicious cooked in liquid - either simply in water; or in fish, chicken or vegetable stock or white wine. Razor-shell clams are excellent steamed.

Scallops can have their shells separated with a knife. Eat as soon as possible after buying. They can be steamed, shallow-fried in butter or oil or grilled. They go well with meaty flavours such as bacon or black pudding. Cook quickly if frying or grilling or steam gently to preserve the rich delicate flavour. They cook quickly so be careful or they'll turn rubbery.

Most shellfish we buy today will have been cleaned and prepared, but it's important to clean them properly. Discard any with broken shells, permanently closed shells or any broken-bodied shellfish.

Lobsters and crabs are cooked live because, once killed, their flesh starts to deteriorate very quickly. Fishmongers and wholesalers will only buy live lobsters to sell, and in turn, chefs and cooks, when cooking fresh lobster, will always choose live specimens for freshness. Lobsters sold in supermarkets in this country are generally sold already cooked as the demand for live lobsters isn't as great as it is abroad.

Many people are appalled that lobsters and other shellfish aren't killed before cooking but the reality is that cooking lobsters live is standard practice. Even though there are pressure groups lobbying the government to legislate on the killing of shellfish, there's nothing required by law in Britain to change cooking methods.

There are three commonly practiced methods of killing lobsters and crabs. One is to place the live lobster or crab head-first into a large pan of boiling water. Another method is to freeze the live lobster or crab so it becomes unconscious and then cook it in boiling water. Third, if you know what you're doing, you can pierce the live lobster with a strong knife in one sharp blow between the eyes which kills it instantly, and then boil it in the usual way. Crabs have two nerve centres so the shell needs to be pierced in two locations.

Always ensure the pan is large enough for the lobster or crab and that the water is boiling thoroughly. If the pan's not large enough it could take longer for the lobster or crab to die.