Complex I (NADH:ubiquinone oxidoreductase) is a membrane-bound, multisubunit enzyme. At present, there is no high-resolution structural model available for complex I and its mechanism of energy transduction is unknown. However, the subunit compositions of complex I from both eukaryotic and prokaryotic organisms have been determined, the sequences of their subunits are known and a coherent picture of the redox cofactors present in complex I has been developed. Here, I aim to describe and examine data relating to the mechanism of complex I and to build a framework to facilitate the discussion of possible conclusions and mechanistic models.