Nothing fascinates me more than the mystery of vertebrate metamorphosis. How does a tadpole turn into a frog? What genes mediate the abrupt transformation of a larval fish or amphibian into its juvenile form? How did metamorphosis evolve? I have studied the biology of vertebrate metamorphosis in fishes and frogs for over 22 years. Metamorphosis is a period during which virtually every organ in the body changes abruptly and dramatically. Metamorphosis in frogs is characterized by three broad categories of development: 1) apoptosis, during which larval-specific structures (e.g. tail, gills) are programmed to die; 2) cell proliferation and differentiation of adult-specific organs (e.g. growth of the limbs); 3) the remodeling of all other organs from larval to adult forms (e.g. the intestine of the herbivorous tadpole remodels to accommodate a carnivorous adult lifestyle). Remarkably, all of these developmental programs are mediated entirely by one molecule: thyroid hormone. As such, amphibian metamorphosis is a naturally-inducible system that is conducive for studying numerous questions relating to the development of virtually any tissue or organ system.