An articular cartilage lesion of the knee is a therapeutic problem and the capacity of cartilage to heal is very limited. Unhealed cartilage lesions are likely to progress to osteoarthritis. Therefore, it is important to diagnose and treat the lesion at an early stage. Autologous chondrocyte transplantation (ACT) is a two-staged procedure that consists of initial arthroscopic evaluation and cartilage harvesting. After isolation and 2 weeks of culturing of the chondrocytes, the implantation can be done in open surgery. Rehabilitation starts the day after surgery and continues over a long period of time; return to competitive sports is possible. The long-term results after ACT are good or excellent in more than 80% of the patients.