Dysplasia of the hips is the most common orthopaedic hereditary disorder in dogs and can result in osteo-arthritis, also called arthrosis or degenerative joint disease. The pathology caused by this multigenic trait is reinforced by mechanisms of transition mutagenesis and by epigenetic changes induced by external factors. Hip dysplasia is diagnosed through radiological techniques, possibly involving distraction, and by genomic markers. The measurement of the Norberg angle is of pivotal importance, and should implement accurate computer-assisted image analysis. Reciever operating characteristic (ROC)-curve statistics reveal the Norberg angle to be the best discriminant between hips with or without dysplasia, based on the FCI criteria for classification. Since the frequency distribution of the Norberg angles is breed-specific, the result of individual dogs must be ranked into a percentile order by breed. Selection of couples for breeding should take the percentile ranking of both parent dogs into account in order to reduce the prevalence of canine hip dysplasia.