Injuries of the medial collateral ligament (MCL) of the knee are easily diagnosed on clinical grounds. These lesions are generally treated conservatively. Radiological examinations are generally unhelpful. However, injuries of the MCL may be associated with those of many other ligaments of the knee in the case of complex strain of the knee so that the clinical diagnosis may be challenging; in some cases, surgical repair of associated lesions is indicated. In these cases of multiple ligamentous injuries, a radiological study of ligamentous abnormalities may be of interest. The anatomical background and the different radiological findings including those of X-rays, US, arthrography and MRI, obtained in patients with injuries of the MCL, are described; radiological differentiation of ruptures of the MCL with or without knee instability is emphasized.