Insect development is primarily governed by two hormones, i.e., ecdysteroids and juvenile hormones (JHs). Larval molt is initiated through the combined action of ecdysteroid and high levels of JH, and the decline in JH levels is necessary to program pupal metamorphosis by the action of ecdysteroid. Thus, JH serves as a critical modulator of ecdysteroid action, orchestrating the timing of molting and metamorphosis in insects. Precise fluctuations in hemolymph JH levels are maintained through the tightly regulated synthesis of JH by the corpora allata (CA) in response to developmental cues. In this chapter, we elucidated the regulatory mechanisms of JH synthesis during the final two larval stadiums based on studies in the silkworm, Bombyx mori, a well-established model species. CA activity is regulated in a stage-specific manner by multiple factors, including ecdysteroids, peptide hormones, and dopamine. In particular, the inactivation of CA, which triggers pupal metamorphosis, is achieved through the coordinated action of three different mechanisms, each operating via a different mode of action.