Oxidative phosphorylation disorders make a contribution of 1 per 10,000 live births in man, of which isolated complex I deficiency is frequently the cause. Complex I, or NADH:ubiquinone oxidoreductase, is the largest multi‐protein enzyme complex of the mitochondrial electron transfer chain. In complex I deficiency, various clinical phenotypes have been recognized, often resulting in multi‐system disorders with a fatal outcome at a young age. Recent advances in complex I deficiency, regarding clinical, biochemical, and molecular aspects are described. However, the genetic causes of about 60% of complex I deficiency remain unclear. As a consequence, further research will be needed to clarify the genetic defects in the remaining cases. Novel strategies in which interesting non‐structural nuclear‐encoded disease‐causing genes may be found, as well as the molecular genetic composition of human complex I, are presented. © 2001 Wiley‐Liss, Inc.