This guidebook describes the classic GR20 trek, a north-to-south traverse of the rugged mountains of Corsica, renowned as Europe's toughest GR long-distance route. Described in 16 stages, some with high and low-level alternatives, the route is roughly 190km in length and takes approximately two weeks to complete. The GR20 climbs high into the mountains. It is a demanding trek suitable only for experienced walkers, and there are several steep 'hands-on-rock' sections calling for a sure foot and a good head for heights. It is possible to stay under a roof every night but many trekkers opt to camp. Step-by-step descriptions of each stage are accompanied by 1:50,000 mapping, together with information on ascent/descent, terrain and the availability of food, water and accommodation/shelter. There is also invaluable practical information such as path conditions, what to take, and getting to/from and around Corsica. Overviews of the history and geology of the island and local plants and wildlife are offered to enhance the trekking experience. Finally, a separate map booklet, included with the guide, provides all the 1:25,000 IGN mapping needed to complete the GR20. Starting in Corsica's northwest, in picturesque Calinzana, the route winds southeast through the heart of the island, finishing on the eastern side, just north of Porto Vecchio in the township of Conca. It showcases Corsica's rich diversity of landscapes and ethereal beauty. Bare rock and sheer cliffs contrast with black sand beaches, alpine pastures, maquis and pockets of forest. There are also opportunities to detour from the route to visit traditional mountain towns and villages for a taste of Corsica's vibrant history and culture and to take in neighbouring summits, including the island's highest peak, Monte Cinto. For those up to the challenge of hiking part or all of the route, the GR20 promises adventure, memorable experiences and rich rewards.