Injury to the knee can damage the lining of the knee's bones, called cartilage. Cartilage injuries cause pain and limit movement, making activities like walking, playing sports, and working difficult. Cartilage cannot repair itself well, so surgery is often needed to repair it. People who have cartilage repair surgery want to return to normal activities after their operation. Doctors and scientists know the operation can reduce pain, but do not fully understand how it affects movement. This research will help us see if knee function gets better after cartilage repair surgery. The results will help doctors and patients understand what to expect from the surgery. It could also uncover common problems after surgery that could be fixed with physiotherapy. This research is important because untreated cartilage injuries can develop into arthritis later in life. Arthritis is a painful lifelong condition that could be prevented by effectively treating the cartilage injury. In this study, adult patients who are waiting to have a type of cartilage repair surgery called 'autologous chondrocyte implantation' (ACI) at one of 7 hospitals will be invited to take part in the study by their surgeon. Patients who decide to take part will be invited to two appointments at their own hospital, where assessments of their knee function will be performed. The tests will assess the knee's movement, and the patient's balance and walking abilities. The first research appointment will take place before the operation, and the second and final appointment will take place 6-months after surgery. The results of this study will help us understand how knee function changes after cartilage repair surgery. The study will take place across 7 ACI centres in England, and is funded by Orthopaedic Research UK and the British Association for Surgery of the Knee.