Animal models of osteoarthritis (OA) provide valuable insight into pathogenetic pathways. Although OA is not an inflammatory disease, synovial activation clearly plays a role. Matrix metalloproteinases 3 (stromelysin) and 13 (collagenase) appear crucial, and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-5-mediated aggrecan cleavage in the interglobular domain is key in OA cartilage damage. Apart from cytokines, chondrocyte derangement or hypertrophy may contribute. Enhanced discoidin domain receptor-2 expression is found in OA models and human OA. Moreover, deficiency of Runt-related transcription factor-2 ameliorates murine OA. The abundant transforming growth factor-beta prevents chondrocyte hypertrophy. Age-related loss of proper transforming growth factor-beta signaling, with a major drop in SMAD-2 phosphorylation, may contribute to hypertrophy and subsequent development of OA cartilage pathology. Finally, transgenic mice show that osteoprotegerin is protective in OA, and treatment studies with recombinant osteoprotegerin have identified chondrocyte apoptosis blocking as an underlying mechanism.