A popliteal cyst, also known as a Baker's cyst, is a fluid-filled sac that forms behind the knee. It occurs when excess synovial fluid accumulates in the knee joint, often due to conditions like arthritis or meniscus tears. The cyst can cause swelling and discomfort, especially when bending the knee. While many people with a popliteal cyst experience mild symptoms, some may feel pain or tightness in the back of the knee. In some cases, the cyst can rupture, leading to swelling and pain in the calf. Treatment options include rest, ice, compression, and, in severe cases, drainage or surgery.