Britain's Chris Froome is looking to win a fourth Tour de France title and third in a row.
The three-week 3,540km (2,200-mile) race started in Germany with a 14km individual time trial. It ventures through Belgium and Luxembourg before heading into France.
The route takes the 198 riders down the eastern side of France to the Jura mountains in the opening week before a rest-day transfer across to the Dordogne in the west.
Two days in the Pyrenees mountains will dominate the second week, while the third week is all about the Alps and what race organisers will hope is a decisive 22.5km individual time trial on the penultimate stage.
Former British cyclist Rob Hayles, who will be BBC Radio 5 live's expert summariser, offers his guide to each of the 21 stages...
Geraint Thomas is the first Welshman to win a stage at the Tour de France
Winner: Geraint Thomas (GB/Team Sky)
Thomas produced a stunning time trial to win the opening stage. The Team Sky rider was not considered among the pre-stage favourites but covered the course in 16 minutes four seconds to claim his first Grand Tour stage and pull on the yellow jersey for the first time. Thomas' team-mate and defending champion Chris Froome finished sixth and gained an early advantage on his general classification rivals. Overall contender Alejandro Valverde (Movistar) was forced to abandon after crashing heavily into a barrier in wet conditions.
One of Marcel Kittel's team-mates was celebrating long before his team leader crossed the finish line in Liege
Winner: Marcel Kittel (Ger/Quick-Step Floors)
Kittel powered home in a star-studded sprint finish to take the stage in Liege. Geraint Thomas held on to yellow by coming home safely in the peloton but there was a scare for Team Sky team-mate Chris Froome, who crashed in the wet 30km from the finish. He resumed and finished the stage in 37th.
Peter Sagan moved fourth overall with his victory
Winner: Peter Sagan (Svk/Bora-Hansgrohe)
World champion Sagan showed remarkable composure to win stage three, triumphing in an uphill sprint after his foot had briefly come loose. Britain's Geraint Thomas continues to lead the race after finishing two seconds adrift, alongside defending champion Chris Froome and the other race favourites.
Cavendish (left) was left with nowhere to go as he collided with the barriers in Vittel
Winner: Arnaud Demare (Fra/FDJ)
Britain's Mark Cavendish accused world champion Peter Sagan of elbowing him as he crashed heavily in a sprint finish on stage four of the Tour de France. The two collided on the sprint finish into Vittel as they competed for the stage win. Arnaud Demare went on to win the stage, becoming the first Frenchman to win a bunch sprint at the Tour since 2006.
Froome said it was "amazing" to be back in the race leader's yellow jersey
Winner: Fabio Aru (Ita/Astana)
Italian Fabio Aru made the decisive break a couple of kilometres from the summit La Planche des Belles Filles to move up to third overall. But Chris Froome put more time into his other rivals by finishing third to swap places with Team Sky team-mate Geraint Thomas at the top of the standings.
Kittel (in blue) draws level with fellow German Andre Greipel (next to him in red) on 11 Tour stage wins
Winner: Marcel Kittel (Ger/Quick-Step Floors)
On a blisteringly hot day, with temperatures on the road reaching 37C, Marcel Kittel sprinted to his second stage win of this year's Tour as the general classification contenders took it easy in the peloton. Kittel edged out Arnaud Demare to win an 11th Tour stage.
Marcel Kittel (in blue) was declared the winner of stage seven after much deliberation
Winner: Marcel Kittel (Ger/Quick-Step Floors)
Marcel Kittel equalled Erik Zabel's German record of most Tour stage wins with his third on this year's race and 12th in total. It took several minutes for the Quick-Step Floors rider to get the nod but he was judged to have beaten Edvald Boasson Hagen by just 6mm. It was a quiet day for the general classification riders with no changes in the standings.
Calmejane battled cramp in the final 5km of the stage
Winner: Lilian Calmejane (Fra/Direct Energie)
Lilian Calmejane delighted the home fans with a second French victory on this year's Tour after attacking on the final ascent and soloing to victory across the 12km plateau to the finish. His win followed a brutal opening two hours of racing with repeated attacks as riders tried to get in the break. Chris Froome said it was "an intense" day as he retained his race lead.
Uran (green) won by a slightly bigger margin than Kittel on stage six as Froome came home third
Winner: Rigoberto Uran (Col/Cannondale-Drapac)
Rigoberto Uran outsprinted a select group of general classification riders, including Chris Froome and Romain Bardet, to win a tortuous day in the Alps that featured seven climbs. The Colombian pipped Warren Barguil on the line, with Froome finishing third to pick up four bonus seconds and extend his overall lead. However, the Team Sky leader lost super-domestique Geraint Thomas, who broke his collarbone in a crash. Australia's Richie Porte, one of the pre-race favourites, also crashed out in a 72km/h accident on the descent of Mont du Chat.
Kittel's win in Bergerac was his 13th stage win at the Tour de France
Winner: Marcel Kittel (Ger/Quick-Step Floors)
The race resumed after the first rest day and once the lengthy breakaway by Elie Gesbert and Yoann Offredo was reeled in on the outskirts of Bergerac, the stage was always going to end in a sprint finish. German Marcel Kittel powered past his rivals to claim his fourth stage win of this year's race, his 13th in total. It took him one beyond Erik Zabel as the German with the most Tour de France wins.
Kittel made it back-to-back victories and five stage wins in this year's race in Pau
Winner: Marcel Kittel (Ger/Quick-Step Floors)
A three-man breakaway of Frederik Backaert, Maciej Bodnar and Marco Marcato attacked early on, with the peloton content to let them go before reeling them in later. However, just as the bunch were about to catch the leading group, Bodnar attacked and stayed out in front until the final 250 metres when he was finally swept up. At that point, German Marcel Kittel underlined his dominance in sprint finishes by easily claiming his fifth stage win of this year's race.
Aru took the yellow jersey for the first time in his career after Froome faltered on the final climb
Winner: Romain Bardet (Fra/AG2R La Mondiale)
On a thrilling first day in the Pyrenees, a group of 12 riders broke away before Britain's Steve Cummings left the rest behind with around 30km to go. Team Sky controlled the tempo on the front of the peloton, catching Cummings in the final 10km and looking to set up leader Chris Froome. However, a brutal summit finish in Peyragudes saw Froome crack inside the final 500m, with Romain Bardet winning the stage and Fabio Aru gaining enough time to take the yellow jersey.
Barguil became the first Frenchman in 12 years to win on Bastille Day
Winner: Warren Barguil (Fra/Team Sunweb)
Report: France's Barguil wins on Bastille Day as Froome stays second
The shortest road stage of this year's Tour saw plenty of attacking early on before Alberto Contador and Mikel Landa broke clear with 60km remaining. The Spanish pair were joined by Nairo Quintana and Warren Barguil on the final climb before a rapid descent into Foix. Barguil proved strongest in the sprint finish, becoming the first Frenchman to win on Bastille Day since 2005. Behind, Chris Froome marked all the attacks by the other overall contenders but did not gain any time on Fabio Aru, who retained the leader's yellow jersey.
It was all smiles for Froome and his team after he reclaimed the yellow jersey at the end of stage 14
Winner: Michael Matthews (Aus/Sunweb)
A stage that in one sense ended as many predicted - with a sprint finish contested between eventual winner Michael Matthews and Belgium's Greg van Avermaet. But in another, it had a very unexpected conclusion, with Chris Froome taking the yellow jersey off Italian Fabio Aru after a dramatic charge up the short but punchy finish in Rodez. Froome, who lost yellow at the end of stage 12, turned a six-second deficit into an 18-second lead.
Mollema broke clear to claim his first Tour stage win as Froome recovered from a mechanical issue to stay in yellow
Winner: Bauke Mollema (Ned/Trek-Segafredo)
A hilly stage saw a large breakaway group go up the road, from which Dutch rider Bauke Mollema attacked with over 30km to go, staying clear to claim his first Tour stage win. Behind, Romain Bardet and his AG2R La Mondiale team-mates put Chris Froome under pressure on the lower slopes of the Col de Peyra Taillade, distancing the race leader. Around this point the Team Sky rider also had a mechanical problem that forced him to swap his rear wheel with team-mate Michal Kwiatkowski, falling almost a minute behind his rivals. However, with the help of his team, Froome chased back and caught the group of favourites before the summit to ensure he retained the yellow jersey at the end of the stage.
Matthews won his second stage of this year's race to cut the gap to Kittel in the green jersey battle
Winner: Michael Matthews (Aus/Sunweb)
Michael Matthews and his Sunweb team-mates drove the pace on the first climb and quickly dropped struggling green jersey leader Marcel Kittel, who never caught back on. Australian Matthews took 20 points at the intermediate sprint and 30 points at the finish as he claimed his second win of the Tour and closed the gap to Kittel in the points classification to 29 points. Crosswinds in the final 20km fractured the leading group, with Ireland's Dan Martin caught behind and losing time, but Chris Froome safely held on to the yellow jersey.
Rob's guide: Two huge mountain passes dominate today's stage and the Col du Telegraphe is no easy climb either. The Col du Galibier is back on the Tour route for a 33rd time and first since 2011. It is the roof of this year's race and the rider who reaches the top first will be rewarded with the Souvenir Henri Desgrange in memory of the Tour's founder. It will be a big day for the general classification contenders.
One to watch: Alberto Contador. The Spaniard has won the Tour twice before and geared up his entire season around this year's race. Is he too old, at 34, to win a third? Expect him to delight fans with his attacks as he dances his way up the Galibier but will it be enough?
Rob's guide: A first finish at the summit of the Col d'Izoard promises to be sensational. There could well be two races going on. The one for the yellow jersey and, with double points on offer, the one for the King of the Mountains. This will be the 35th ascent of the legendary Alp and it could be crucial to determining the winner.
One to watch: Pierre Rolland. The Frenchman won the white jersey as best young rider in 2011 but hopes he would be the first home rider to win the race since Bernard Hinault in 1985 have long faded. He loves the high mountains though and at 30 years old knows opportunities to add to his two Tour stage wins are fast running out.
Rob's guide: The race tumbles out of the Alps and heads southwards towards the Mediterranean on the longest stage of the race. Will the sprinters be bold enough to ask their teams to ride for them in the hope of setting up a sprint finish? Or will everyone be too tired to care and allow a breakaway to succeed? Or will a puncheur use the Col du Pointu to ride clear? I reckon it could be the latter.
One to watch: Steven Cummings. The Briton has won stages in each of the last two Tours, could he make it a hat-trick?
Rob's guide: Race organisers will be hoping that Chris Froome needs to make up time on some on his rivals because if he arrives in Marseille with any kind of advantage, it is race over. A bit of a hill two-thirds of the way into the test is not stiff enough to create any significant gaps. If nothing else, it should provide good images for the television.
One to watch: Chris Froome. Will this be a lap of honour around Marseille for the Team Sky leader? If it is, don't back against him extending his advantage and chalking up another stage win.
Rob's guide: And so to the now traditional early evening dash round Paris. The jerseys will have been decided and champagne will have been sipped on the road in from Montgeron - where the Tour de France started in 1903. Tommy Voeckler may well be allowed a lap out front on his own in recognition of his efforts before the serious racing begins for a coveted stage win on the Champs Elysees.
One to watch: Andre Greipel. The German is chasing a third successive victory on the cobbles. He has won a stage on every Grand Tour he has ever raced in. Will he still be searching come Paris?
Yellow - the coveted maillot jaune is worn by the rider how has taken the least cumulative time to complete the race after each stage and thus the overall Tour winner. The winner receives 500,000 euros (£440,000).
Green - worn by the leader of the points classification. The maillot vert rewards consistently high finishes with points being awarded for intermediate sprint and end-of-stage placings. The winner receives 25,000 euros (£22,000).
Polka dot - the maillot a pois is worn by the 'King of the Mountains'. Points are awarded on categorised climbs with the harder ascents offering more points. The winner receives 25,000 euros (£22,000).
White - the maillot blanc has been awarded every year since 1975 and is open to riders aged 25 and under. If a rider is leading any other classification, that jersey takes precedence. The winner receives 20,000 euros (£17,600).
Each stage winner receives 11,000 euros (£9,680).
There is a 'most combative rider' prize, selected each day by a jury of four cycling experts. The rider is denoted by a red number on the following stage and the race's most combative rider - super-combatif - is decided after the final stage and awarded 20,000 euros.
There is also a 50,000 euro (£44,000) prize for the best team performance, which is calculated by adding up the times of the first three riders on each team.