The osteochondroses are a heterogeneous group of lesionsoccurring in growth (epiphyseal) cartilage of immature animals and are characterized by focal or multifocal delay in endochondral ossification, which is the process by which growth cartilage is converted to bone. The sites of occurrence of these lesions includethe growth plate (physis) and the epiphyseal cartilage of the articular-epiphyseal cartilage complex (AE complex). The lesions are common, often occur in bilaterally symmetrical sites, and represent an important orthopedic entity that has a number of different clinical manifestations in pigs, dogs, horses, cattle, and poultry. The hallmark of the uncomplicated gross lesion of osteochondrosis is focal or multifocal retention of growth cartilage due to its failure to become mineralized and replaced by bone by endochondral ossification. In the growth plate, this is due to an accumulation of viable hypertrophic chondrocytes, whereas in the AE complex, it is due to necrosis of epiphyseal cartilage.