A Baker's cyst is a protrusion of the joint capsule in the knee joint that is filled with synovial fluid. The protrusion can be found in the popliteal fossa and, depending on the size of the swelling, can be seen and felt. The Baker's cyst is not an independent disease, but occurs as a secondary symptom of another knee disease (e.g. knee osteoarthritis). Due to degenerative damage to the knee joint, the body tries to protect the knee joint by increasing the production of synovial fluid. As the joint capsule cannot collect the increased joint fluid, the joint capsule bulges outwards. The symptoms and the volume of the Baker's cyst can vary. Small cysts can even be symptom-free. However, typical symptoms include visible swelling in the hollow of the knee, a feeling of tightness in the knee or calf, tenderness and restricted movement in the knee joint. As the Baker's cyst is a consequence of a previous illness, possible complaints can arise from the previous illness or the Baker's cyst.