In brief: The effect of knee joint hypermobility on knee injury is not fully clear. Inherited flexibility probably plays little role in the causes of most knee injuries. The exception is patellar dislocation, which is strongly associated with hypermobility. Discussions of hypermobility often reflect confusion over the difference between flexibility and laxity. In general, flexibility refers to a joint's range of motion and is a function of muscle and tendon tightness, while laxity refers to joint distraction and shear and is a function of ligament tightness. Research on hypermobility has focused on flexibility; the relationship between injury and joint laxity (ligament stability) has not been studied prospectively. Such research could yield information useful for injury prevention.