Zwift currently has 3 challenges available to users. A challenge is a cumulative effort over time to complete a specific goal.
Challenges are selected from the 'Pause' menu while in-game and look like one of the 3 images to the right. The challenge needs to be pre-selected in order for your cumulative effort to be counted. Once selected, you don't need to do anything else as all your effort will go towards that challenge. You can change challenges without losing your current accrued effort but the new effort will go towards the selected challenge.
Climb Mt Everest Challenge[]
Trek Emonda - Climb 8848m Everest Challenge
Tron Bike - Climb 50,000m Everest Challenge
This is the most difficult of the challenges and will give you the best reward (unlock of the Zwift Concept Z1 bike aka Tron). This challenge actually has two parts so don't stop after you reach the first milestone. It is highly recommended for new Zwifters to select this challenge first and even before you start your first ride in Zwift. The challenge is to climb 8848m (29,029ft - height of Mt Everest). All your cumulative climbing effort will count towards the goal. Once you reach 8848m, you will unlock a virtual Trek Emonda bike. But don't stop now, leave the challenge selected and continue climbing until you reach 50,000m (164,042ft). Once you hit that goal, you will unlock the Zwift Concept Z1 bike known as the Tron bike. This is the only way to acquire the Tron bike.
The challenge shows your progress along the way with some animation of a bike climbing Everest and further on into the stratosphere. For the hardcore climbers, there is a separate achievement called vEveresting. This is where you climb the vertical distance of 8848m all in one ride in Zwift. There is an extra credit achievement badge for this effort.
Ride California Challenge[]
Specialized Venge 2015 - Cycle 1283km Ride California Challenge
The Ride California Challenge is to cycle 1283km (797.2 miles). Once selected, all your cumulative effort will count towards completing this challenge. The challenge shows your progress along the way with some animation of a bike traversing various points from San Diego to San Francisco. Once you've completed the challenge, you will unlock the Specialized Venge 2015 virtual bike (prior to v1.54 the Specialized S-Works Tarmac was the reward). This is a decent all-arounder bike for new Zwifters.
Pinarello F8 Dogma - Cycle 2000km Tour Italy Challenge
Tour Italy Challenge[]
The Tour Italy Challenge is to cycle 2000km (1242.7 miles). Once selected, all your cumulative effort will count towards completing this challenge. The challenge shows your progress along the way with some animation of a bike traversing various points in Italy. Once you've completed the challenge (actually at about 90% complete when you reach Treviso), you will unlock the Pinarello F8 Dogma virtual bike. While a nice looking bike, it's not terribly fast in-game.