By Dr. Jeffrey Peng, MD · Published March 4, 2026 · 6 min read Watch the Full Video A Baker's cyst — also called a popliteal cyst — is a fluid-filled swelling that develops in the back of the knee. Named after the British surgeon Dr. William Baker, who first described the condition, it can range from a painless incidental finding on an MRI to a significant source of discomfort that limits everyday activities like bending, squatting, and walking. In my practice, Baker's cysts