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 and creates a bulge. This cyst can cause discomfort, stiffness, or swelling in the knee area. While a Baker's cyst is usually not harmful, it can sometimes lead to complications, such as pain or restricted movement. Treatment options may include rest, ice, compression, and elevation (RICE), as well as physical therapy or, in some cases, aspiration to remove the fluid.