A mandatory carrier bag charge has been introduced in Scotland for the first time.
Shoppers will now have to pay a minimum of 5p per bag, with many retailers donating proceeds to good causes.
In May, MSPs voted to bring in the new regulations in a bid to tackle Scotland's litter problem, but the fee does not just apply to plastic bags.
It has also been imposed on single-use carriers made from eco-friendly materials.
The minimum charge applies to all retailers, not just supermarkets.
Wales became the first part of the UK to introduce a minimum charge for single-use carrier bags in 2011, followed by Northern Ireland last year.
What does the new bag charge mean?
What are the reasons behind the charge, how will retailers will be affected, where will the money go and exactly which bags will customers be paying for?
The Scottish government has estimated that 800 million single-use bags are given out by supermarkets every year in Scotland alone.
More than 160 retailers including Marks and Spencer, McDonald's and The Co-operative Group have registered for Zero Waste Scotland's Carrier Bag Commitment, launched earlier this year.
This means organisations have agreed to donate the net proceeds from the charge to good causes, which may include environmental causes.
Environment Secretary Richard Lochhead said: "I'm extremely proud that this landmark legislation is now in force.
"Our carrier bag addiction is symptomatic of our throwaway culture and has serious implications for the environment.
"Huge numbers of these bags end up as litter, blighting our communities and clogging up our seas and natural habitats, affecting many sorts of wildlife and marine species in particular.
"We want that to change and for people to stop and think about whether they really need to take another bag.
"Alternatives like bags-for-life are easy to get and are much more sustainable."
The Marine Conservation Society said the Scottish move was "a major step forward in tackling a problem that causes so much harm to marine wildlife".
The society has argued that it will reduce harmful litter on beaches and in the sea, and prevent many wildlife entanglements and choking deaths.
Pollution programme manager Laura Foster said: "We look forward to seeing the results in Scotland following the great example that has been set in Wales and in Northern Ireland.
"A 5p charge on all single-use carrier bags, regardless of the material they are made from, is a really encouraging step forward and millions of bags will now not end up in Scottish waters thanks to this decision by Holyrood."
Environmental group WWF Scotland also welcomed the mandatory fees.
Director Lang Banks said: "We know that in other parts of the world, charges for plastic bags have led to dramatic reductions in their use, as well as positive changes in consumer behaviour.
"A great example is Denmark, which introduced a charge in 2003, and now has the lowest plastic bag use in Europe, using four plastic bags per person per year.
"At present Scots consume nearly 800 million carrier bags every year, with millions ending up in landfill, polluting our environment and threatening wildlife."