This chapter discusses the insect CYP genes and P450 enzymes. Cytochrome P450, or CYP genes, constitutes one of the largest family of genes, with representatives in virtually all living organisms, from bacteria to protists, plants, fungi, and animals. The multiple functions of insect P450 enzymes, their complex biochemistry, and their toxicological and physiological importance are presented. Physiological functions are not restricted to one branch of the P450 evolutionary tree, and the ramifications that seem typical of “environmental response genes” are found in both microsomal and mitochondrial P450 encoding genes. The CYP3 and CYP4 clans seem the most diverse, and the smaller CYP2 and mitochondrial CYP clans seem to have proportionally more genes found as orthologs across species. There are many physiological functions for P450s that just are not known or suspected. CYP4C7 was shown to be selectively expressed in the corpora allata and to metabolize JH and its precursors to new metabolites.