A Baker's cyst is a pocket of fluid that forms a lump behind the knee. Baker’s cyst is also referred as popliteal cyst. Baker’s cyst is caused when a joint fluid is pushed together in a tissue behind the knee. When this fluid fills in tissue and bulges out, it is called as cyst. Baker’s cyst can be diagnosed by MRI scan. This can be treated by draining the fluid with needle. We can also use steroids to reduce the swelling. Sometimes cyst may be removed by surgery.