Baker's Cyst: This is a fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee. It is the most common cause of pain behind the knee and is often associated with knee arthritis or a knee injury.

Hamstring Strain: A common injury, especially in athletes, where the muscles at the back of the thigh are stretched or torn, leading to pain behind the knee.
Popliteus Tendinitis: Inflammation of the tendon that connects the popliteus muscle to the bone, causing pain at the back of the knee.
Cystic Meniscal Tears: Tears in the cartilage of the knee joint can cause fluid-filled cysts to form, leading to pain behind the knee.

Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the leg, which can be life-threatening if it breaks loose and travels to the lungs. It often presents with swelling, redness, and pain in the leg, including behind the knee.
Popliteal Artery Aneurysm: A bulge in the artery behind the knee, which can lead to clotting or rupture, causing severe pain and potentially life-threatening complications.
Osteosarcoma or Other Bone Tumors: Although rare, bone cancers can cause pain behind the knee and must be considered to ensure timely diagnosis and treatment.

Ganglion Cyst: A lump under the skin that occurs when fluid leaks from a joint or tendon, sometimes found behind the knee.
Lipoma: A benign tumor made up of fat tissue, which can occur almost anywhere, including behind the knee, and may cause discomfort or pain if it presses on nearby structures.
Neurogenic Tumors: Tumors that arise from nerve tissue, such as neurofibromas or schwannomas, can cause pain behind the knee if they compress or infiltrate nearby nerves.