Juvenile hormones (JHs) are synthesized in a pair of tiny endocrine organs, the corpora allata (CA), through 13 enzymatic steps. This JH biosynthetic pathway is conventionally divided into two phases, the early mevalonate pathway and the late JH-branch pathway. The early pathway produces farnesyl pyrophosphate (FPP) from acetyl-CoA, and the late pathway catalyzes the conversion from FPP to JHs. There are several variations in the JH biosynthetic pathway depending on insect groups. In lepidopteran insects, the order of the final two steps of the late pathway is reversed, and they utilize propionyl-CoA as well as acetyl-CoA as starting molecules to produce ethyl-branched JHs. Although many insects use single epoxidized JHs, some of the Hemiptera and higher Diptera also use double epoxidized JHs with an additional epoxidation possibly catalyzed by unknown epoxidases. Enzymes catalyzing 13 steps of JH biosynthesis have been identified. Although most of them are found in all insects studied, orthologs of the enzymes catalyzing the first three steps of the late pathway are not always found, or genes of their most similar homologs are hardly expressed in the CA in some species. In these species, other pathways from FPP to farnesoic acid are proposed.