The initial radiological assessment of knee osteoarthritis includes the evaluation of the osteochondral destruction, collateral ligament laxity and the determination of the position of the patella in both the axial and the sagittal planes and the femorotibial angulation in the coronal plane. The different types of knee prostheses are described. Post-operative radiological features of knee arthroplasties are discussed including normal findings and criteria of prosthesis instability and loosening.