Bursitis of the knee: This condition, particularly prepatellar or infrapatellar bursitis, is a common cause of pain in the front or back of the knee, especially when kneeling. The bursae can become inflamed due to repetitive pressure or friction, leading to pain and swelling.

Tendinitis: Inflammation of the tendons around the knee, such as the patellar tendon, can cause pain at the front or back of the knee, especially with activities like kneeling.
Meniscal tears: Tears in the cartilage of the knee joint can cause pain, especially with twisting or bending movements, including kneeling.
Ligament sprains: Sprains to the ligaments surrounding the knee can cause pain and instability, potentially worsening with kneeling.
Osgood-Schlatter disease: This condition, common in adolescents, involves inflammation of the patellar tendon at its insertion on the tibia, causing knee pain that can be exacerbated by kneeling.

Deep vein thrombosis (DVT): Although less common, DVT can cause pain in the back of the knee, especially if the clot is located in the popliteal vein. This condition is a medical emergency due to the risk of pulmonary embolism.
Popliteal artery aneurysm: A rare but potentially life-threatening condition where an aneurysm in the popliteal artery can cause pain behind the knee, among other symptoms.
Infection (septic arthritis or osteomyelitis): Infections within the knee joint or bone can cause severe pain, swelling, and redness, and require urgent medical attention.

Cystic lesions (e.g., Baker's cyst): Although not typically painful unless ruptured, a Baker's cyst can cause discomfort or pain behind the knee, especially if it becomes large or ruptures.
Tumors (benign or malignant): Rarely, tumors in the soft tissues or bones around the knee can cause pain, especially if they impinge on nerves or other structures.
Neurogenic causes (e.g., sciatica, radiculopathy): Pain referred from the lower back or due to nerve compression can sometimes be felt in the back of the knee, though this is less common.