It’s a challenge as old as the movies themselves: Hitting the gym to bulk up, add muscle, and get ridiculously shredded for a film role.

Stars have pulled off incredible body transformations over the years. Some Hollywood icons—like Will Smith, Robert De Niro, Brad Pitt, and Bradley Cooper—transformed from leading-man lean to Muscle Beach-built to portray muscular, imposing characters. Other guys—like Arnold Schwarzenegger, Sylvester Stallone, Dwayne "The Rock" Johnson, and Dolph Lundgren—added even more beef to their already-muscular frames, pushing themselves to the limits of human muscularity.

Some stars had the help of trainers to get them over the top. Trainer or no trainer, it meant getting into the gym and lifting some heavy stuff. No matter how they did it, these stars all impressively muscled up for their roles.

Here’s a look at the top 25 most intense Hollywood bulk-ups of all time.