Most of the energy produced in the form of ATP is produced in the mitochondria. Cellular processes involved in energy production and conversion to more useful forms require highly complex molecular machinery. Decades of research into the structure and function of respiratory complexes have given valuable insights on the mechanism of coupling electron transfer to the translocation of protons but there are still many unresolved questions. The aim of this review is to describe the process of oxidative phosphorylation and to outline all enzymes involved. New insights into the structure, mechanism and functions of individual complexes are included. Also new studies of specific interactions of respiratory complexes are described. All the research on the structure, composition, mechanisms and conditions of assembly of supercomplexes indicate their great importance in the function of mitochondria. Supercomplexes have major roles in the regulation of respiratory activity and generation of reactive oxygen species. Also they are important for the correct assembly of complex I and have a large impact on the morphology of the inner mitochondrial membrane.