The IMSA WeatherTech SportsCar Championship traces its roots to the IMSA GT Championship, which began in 1971. Top-level North American sportscar racing was divided for much of the 2000s between the American Le Mans Series and the Rolex Sports Car Series (Grand-Am). The two were merged in 2014 to form the United SportsCar Championship — subsequently renamed the IMSA SportsCar Championship in 2016. WeatherTech has served as title sponsor since that rebranding.
The series is sanctioned by the International Motor Sports Association and is considered the pinnacle of North American sportscar competition, attracting major manufacturers, elite driver lineups, and both endurance and sprint races. The season opens with the Rolex 24 at Daytona and concludes with the Petit Le Mans, with the Michelin Endurance Cup recognising teams across the five longest events on the calendar.
GTLM was a continuation of the American Le Mans Series GT class. It utilised cars homologated to the ACO's GTE specification — the same technical framework used in the FIA World Endurance Championship's GTE Pro category — and competed from the series' inaugural 2014 season through to the end of 2021. The class required two drivers and was unrestricted in terms of FIA driver rating, meaning full factory lineups with platinum-rated professionals were the norm.
GTLM attracted deep factory investment from manufacturers including Chevrolet (Corvette), Porsche, BMW, Ferrari, Ford (with the GT supercar), Dodge, and Aston Martin at various points. The Ford GT programme, launched in 2016 to coincide with the 50th anniversary of Ford's Le Mans victories, was particularly high-profile and achieved class wins at both Le Mans and Daytona. Corvette Racing was the most consistent presence throughout GTLM's existence, having competed under various IMSA series structures dating back to the early 2000s.
The class ran in close parallel with the WEC GTE Pro regulations, enabling manufacturers to develop a single car platform for deployment across both series, with the Le Mans 24 Hours as the shared prestige target.
GTLM grid sizes contracted sharply in the final seasons. By 2021, manufacturer retirements and the global disruption caused by the COVID-19 pandemic had reduced the class to a skeleton entry. The FIA and ACO's decision to wind down GTE Pro in favour of the Le Mans Hypercar class from 2023 removed the shared regulatory rationale, and GTLM was formally retired after the 2021 season.
GTD Pro was introduced in 2022 as GTLM's successor. Rather than GTE, it uses FIA GT3 specifications — the same platform as the existing GT Daytona (GTD) class — but without the mandatory amateur driver requirement that applies in GTD. This allows factory-supported teams to field full professional lineups under a cost-controlled, widely available GT3 technical formula.
The shift to GT3 specifications lowered the barrier to entry compared to the bespoke GTE programme while maintaining a meaningful factory presence. Porsche, BMW, Ferrari, Corvette, Aston Martin, Mercedes-AMG, and Lamborghini have all fielded entries in GTD Pro since its introduction.
The broader IMSA WeatherTech championship features multiple concurrent classes. Above the GT tiers sit the GTP (Grand Touring Prototype) class — the flagship category from 2023 using LMDh and Le Mans Hypercar machinery — and the LMP2 class. The GT Daytona (GTD) class runs identical GT3 machinery to GTD Pro but mandates at least one silver- or bronze-rated amateur driver per team, maintaining a pro-am character distinct from GTD Pro's professional focus.
The Michelin Endurance Cup, which encompasses the Rolex 24 at Daytona, 12 Hours of Sebring, 6 Hours of Watkins Glen, the SportsCar Endurance Grand Prix, and the Petit Le Mans, provides a championship-within-a-championship for endurance specialists. The GT classes' competitive intensity at these marquee events — particularly at Daytona and Sebring — has consistently produced close manufacturer battles and high-profile driver appearances from international racing stars.