There have been significant advances in our understanding of the contribution of mitochondria to basic cellular function such as energy supply, calcium homeostasis and, more recently, programmed cell death. Mitochondria now appear to play an important role in the final common pathway leading to apoptosis. Study of inborn errors of the respiratory chain is now focussed on understanding pathogenesis, in particular the role of the cell nucleus in determining the expression of mitochondrial DNA mutations. Respiratory chain deficiencies induced by exogenous or endogenous toxins are important in the aetiology and pathogenesis of certain neurodegenerative diseases such as Parkinson's disease and Huntington's disease. A potential role for inborn mitochondrial defects in these disorders has not yet been defined but is currently attracting interest.