This chapter will discuss the knee, the largest synovial joint in the body. It will cover the normal bony anatomy and geometry of the knee and how this gives rise to its complex biomechanics which go beyond a simple hinge. The factors stabilising the knee during its motion will be discussed and how these stabilisers have both primary and secondary restraint functions. It will also discuss our current understanding of pathophysiology of the most common degenerative disease affecting the knee, osteoarthritis as well as its clinical and radiological features, with a brief discussion of the various management options available.