The aetiology of degenerative joint disease is multifactorial, but one main cause is overloading (mechanical stress). While until recently it was well accepted that this represented primarily a disorder of cartilage with reactive subchondral changes, there is now some evidence that it might be primarily a subchondral problem with secondary changes in the articular cartilage. Early subchondral changes include redistribution of blood supply with marrow hypertension, oedema and probably micro-necrosis. These findings are very similar to those in avascular necrosis of bone and raise the question of a vascular aetiology. While these first reports need further proof, it seems clear that the articular cartilage and subchondral regions are one functional unit, in which the subchondral region is more stress sensitive. Recently described channels connecting these two regions strengthen this opinion. These new concepts are exciting and may make a major impact in the near future on the management of and research into degenerative joint disease.