In the complex biomechanics of the knee joint, the synovial membrane plays a crucial role. This chapter provides a thorough exploration of synovial pathologies of the knee, focusing on disruptions within this critical component. It discusses common conditions such as tumors of the synovial tissue, rheumatic diseases affecting the synovial membrane, microcrystalline synovitis, and septic synovitis by emphasizing key aspects such as etiology, clinical presentation, diagnostics, and modern treatment strategies, with a specific focus on surgical techniques in rheumatic diseases. Clinical manifestations generally include knee pain, swelling, stiffness, and functional impairment. The fundamental diagnostic tools encompass physical examination, blood tests, arthrocentesis, imaging, diagnostic arthroscopy, and histopathology. Arthroscopic or open synovectomies are the main surgical options in knee synovial pathologies. Early recognition and a comprehensive approach are crucial for managing these conditions and preserving the health of the knee joint.