Baker's cyst is a fluid collection behind the knee. This cyst may be formed by the connection of a normal bursa (a normal lubricating fluid sac) with the knee joint. This type is more common in children.

The condition can also be caused by the herniation of the knee joint capsule out into the back of the knee, which is more common in adults. This type of Baker's cyst is commonly associated with a tear in the meniscal cartilage of the knee. In older adults, this condition is frequently associated with degenerative arthritis of the knee.

Nearly one half of Baker's cysts are found in children, where they appear as painless masses behind the knee that are more obvious when the knee is fully extended. A large cyst may cause some discomfort or stiffness but generally has no symptoms. Baker's cysts usually disappear spontaneously, but the time in which they do so is variable.