Genu valgum is commonly referred to as "knock knees", a condition where the knees angle in and touch when the legs are straightened. Persons with severe valgus deformities are typically unable to touch their feet together while simultaneously straightening the legs.

Mild genu valgum is relatively common in children up to two years of age, and is often corrected naturally as children grow and develop. However, the condition may continue or worsen with age, particularly when it is the result of a disease, such as rickets or obesity. Idiopathic is the term used to describe genu valgum that is congenital or has no known cause.