There are many examples of oxidative enzymes containing both flavin and heme prosthetic groups that carry out the oxidation of their substrate. For the purpose of this article we have chosen five systems. Two of these, the L-lactate dehydrogenase flavocytochrome b(2) and cellobiose dehydrogenase, carry out the catalytic chemistry at the flavin group. In contrast, the remaining three require activation of dioxygen at the heme group in order to accomplish substrate oxidation, these being flavohemoglobin, a nitric oxide dioxygenase, and the mono-oxygenases nitric oxide synthase and flavocytochrome P450 BM3, which functions as a fatty acid hydroxylase. In the light of recent advances we will describe the structures of these enzymes, some of which share significant homology. We will also discuss their diverse and sometimes controversial catalytic mechanisms, and consider electron transfer processes between the redox cofactors in order to provide an overview of this fascinating set of enzymes.