In insects molting and metamorphosis are primarily under the control of two insect hormones, ecdysone and juvenile hormone (JH). Physiological and biochemical studies of insect hormone metabolic pathways suggested the involvement of P450 (CYP) enzymes in the pathways, but molecular details of the enzymes were unclear. In recent years, the genome information and studies using molecular biology and genetics have allowed us to understand enzymes in the ecdysteroid and JH metabolic pathways. Genome sequencing has been accomplished in several insect species, and has shown the presence of 36–180 CYP enzymes. To date, six and one CYP enzymes have been revealed in the biosynthesis and inactivation pathways of 20-hydroxyecdysone (20E), respectively. In the 20E biosynthetic pathway, correlation among the enzymes, substrates and metabolites is elucidated in the late steps, but the enzyme(s) and intermediates in the early steps have not been fully understood and are referred to as the ‘Black Box’. The gene expression of some CYP enzymes in the 20E biosynthesis is modulated by neuropeptides and JH. Furthermore, involvement of a CYP enzyme is found in both JH biosynthesis and inactivation pathways. Thus, recent studies have shown the importance of CYP enzymes in insect hormone metabolisms.