A Baker’s cyst, also called a popliteal cyst, is a build-up of joint fluid that sits behind the knee. It is not a separate injury on its own. It is a sign that something inside the knee is irritating the joint and causing excess fluid to be produced. The most common drivers are things like a meniscal tear, knee osteoarthritis, ongoing joint irritation, or swelling after an injury. The fluid has to go somewhere, so it tracks backwards into the space behind the knee and forms the cyst. That tight,