The causes of motor neuron death in amyotrophic lateral sclerosis (ALS) are so far unknown. The involvement of mitochondria in the disease was initially suggested by ultrastructural studies. More recently these observations have been supported by studies of mitochondrial function in ALS. Alterations in the activity of complexes which make up the mitochondrial electron transport chain have been recorded as well as mutations in the mitochondrial genome. The calcium buffering function of the mitochondria may also be affected in the disease. This review will discuss how mitochondrial dysfunction could be of relevance in ALS and the evidence that an alteration of mitochondrial function is a feature of the disease. The way in which the involvement of mitochondria fits with other aetiological hypotheses for ALS will also be discussed.