An IMSA race weekend brings all four classes together on one track, so a single race has a winner in each class while the faster cars constantly pick their way through slower traffic.
The weekend builds through practice sessions to qualifying, which is run class by class rather than all together: GTP gets its own timed segment and the other classes theirs, with the fastest lap in each class taking that class's pole. The grid is then set by qualifying order, fastest overall at the front.
Race length is what varies most. Many rounds are shorter sprints, while the marquee events are full endurance races — the Six Hours of Watkins Glen, the 12 Hours of Sebring, the 10-hour Petit Le Mans and the 24-hour Rolex 24 At Daytona.
Every car is shared by a crew of at least two drivers, rising to three or four for the big enduros. Drivers hand over during pit stops, which also cover fuel and tyres. At the five longest races, teams chase their class championship and the separate Michelin Endurance Cup at the same time. For the scoring detail, see how IMSA scoring works.