A Baker's Cyst is a fluid-filled sac that forms behind the knee, often due to swelling from conditions like arthritis or a knee injury. It occurs when excess synovial fluid, which lubricates the joint, accumulates in the popliteal bursa, leading to a noticeable bulge. While a Baker's Cyst can cause discomfort or tightness, many people experience no symptoms. Treatment may include rest, ice, compression, and elevation (RICE), or in some cases, draining the cyst. Addressing the underlying cause, such as osteoarthritis, is also important for long-term relief.