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

Hamstring Strain: Strains to the hamstring muscles can cause pain in the posterior aspect of the knee, especially during activities that involve bending or straightening the knee.
Gastrocnemius Strain: The gastrocnemius muscle is located at the back of the lower leg and can cause posterior knee pain if strained, particularly during sudden stops or changes in direction.
Popliteus Tendinitis: Inflammation of the popliteus tendon, which is located at the back of the knee, can cause pain in this area, often related to overuse or direct trauma.
Meniscal Tears: Tears in the menisci, which are cartilage structures in the knee joint, can cause pain in various locations around the knee, including the posterior aspect.

Deep Vein Thrombosis (DVT): Although less common, DVT can present with posterior knee pain and swelling, and it is crucial to diagnose promptly due to the risk of pulmonary embolism.
Neoplastic Processes: Tumors, either benign or malignant, in the bones, soft tissues, or nerves around the knee can cause posterior knee pain and must be considered to avoid delayed diagnosis.
Infectious Processes: Infections such as osteomyelitis (bone infection) or septic arthritis (joint infection) can cause severe pain and require urgent medical attention.

Cystic Lesions of the Bone: Uncommon bone lesions like unicameral bone cysts or aneurysmal bone cysts can cause pain in the posterior knee area.
Neurogenic Causes: Conditions affecting the nerves, such as a lumbar radiculopathy or peripheral neuropathy, can sometimes present with pain referred to the posterior knee.
Vascular Causes: Other than DVT, vascular issues such as popliteal artery entrapment syndrome can cause posterior knee pain, especially during exercise.