Knee pain can stem from various conditions and injuries. Common causes include osteoarthritis (wear and tear of the cartilage), rheumatoid arthritis (an autoimmune disorder affecting the joints), meniscus tears, ligament injuries (such as ACL or MCL tears), and patellar tendinitis. Other factors like bursitis, gout, and overuse can also contribute to knee pain. Identifying the exact cause is essential for effective treatment.