The Tour de France is the world's biggest annual sporting event.
Nearly 200 cyclists race over 2,000 miles in just 23 days.
The reigning champ is Great Britain's Geraint Thomas. Geraint made his name at the Olympics Games - he won gold in the team pursuit in 2008 and 2012.
Since 2007 he's also raced in the Tour de France but 2018 was the first time he's won it.
He's one of the favourites to do well again, although another famous British cyclist, Chris Froome (who won the race in in 2013, 2015, 2016 and 2017), is out after a serious cycling accident earlier this year.
So do you know your yellow jersey from your polka dot? Your prologue stage from your flat?
If not, Newsround is here to help.
Basically, it's a huge bike race around France that takes place every summer.
Millions of people line the route that's made up of 21 stages raced over 23 days - that means they only have two days off to rest. Ouch!
Twenty-two teams from right across the world are involved in the Tour, with each having up to nine riders.
The riders average around 25mph over the entire course but at some points they'll go a lot faster than that.
Every wondered why the riders are so thin? Well, it's fair to say it's a good way to keep fit.
If you finish the gruelling Tour, you could burn off up to 118,000 calories. That's the equivalent of 26 chocolate bars per day!
The race is broken up into 21 different parts or 'stages' and it isn't just flat roads.
The Tour takes the riders up into the mountains too.
The different stages have different names:
Prologue: Each rider races against the clock in a short (usually under six miles) time trial.
Flat: Despite the name, it doesn't necessarily mean it's perfectly flat. Usually it's big packs of competitors riding together in a big group or 'peloton' for around 125 miles. These end in one of two ways - a 'breakaway' victory by an individual or small group, or typically with a hair-raising bunch sprint.
Time trial: A race against the clock. Similar to a prologue but a little longer. These are shorter stages of around 30 miles (as opposed to 100-125 miles). Sometimes riders do it by themselves, sometimes they ride as a team
Mountain: These come in all shapes and sizes, climbing from sea level to 2,000 metres sometimes more than once in a day. Tough!
The race was started by Henri Desgrange as a publicity stunt back in 1903.
He came up with the crazy idea of a bike race around France in order to raise the profile of L'Auto, the newspaper he worked for.
The first race was a huge success with 60 riders covering an amazing 1,500 miles.
The Tour is now like one big race with lots of smaller races going on inside it.
Riders might race for each stage win or to earn the right to wear one of the famous jerseys...
The green jersey is the points prize. You get points for being one of the first riders over the line on each stage. It's usually won by a sprinter.
But the tour isn't just about reaching the highest speeds - it's also known for its uphill struggles. The riders climb thousands of metres up Alpine mountains, with the best rider on those stages winning a snazzy red and white polka dot jersey.
The white jersey is given to the best young rider - that's someone who's under 25 years old.
But the prize everyone wants is the yellow jersey. If you are wearing it, you are the overall race leader on total time since the start of the Tour.
Tour champions are often strong at everything - climbing, sprinting and time-trialling.
No. It's a big team event but the team leader is very important. The team leader is usually the strongest rider.
The other team members are traditionally called domestiques, from the French word for servants.
These riders work hard in every stage to protect their team leader.
They even have to go and fetch water for him (and everyone else) if he needs a drink.
If he gets a puncture, they will wait while the team mechanic changes his wheel.
The star-man will ride behind his team mates - they protect him from the wind which makes it easier for him and he gets less tired.
Their reward is a share of the prize money and the glory of contributing to a successful team.
When the race is going on, lots of riders often ride in a big group called the peleton.
This is when they all ride together in formation to save energy - a bit like a flock of birds.
No - it regularly nips into other countries that border France and sometimes the race even starts in another country.
The first two days of racing, the Grand Depart, are held in a new location every two years.
It was in London in 2007 and in 2014 the Tour started in Yorkshire.
Since 2014, Yorkshire has hosted its own cycling race inspired by hosting the Grand Depart, called the Tour de Yorkshire, which now takes place every year.