Phase polyphenism is an adaptive phenomenon in which some traits vary continuously in response to population densities. In locusts, two extreme phases, solitarious and gregarious, that occur at a low and a high density, respectively, are known and intermediate forms called transient phase also occur in a transition between the two phases or at an intermediate density. Establishment of an albino strain of Locusta migratoria led to the discovery of a neuropeptide, corazonin that is involved in the control of the expression of some phase-related traits in this and another locust, Schistocerca gregaria. This paper describes a summary of phase polyphenism research with a particular emphasis on recent studies about the roles of corazonin in locusts. In L. migratoria, injection of the neuropeptide causes albino nymphs to express various body colors often observed in not only solitarious but also gregarious forms irrespective of the rearing density. Both juvenile hormone and corazonin are necessary to express the green solitarious form normally observed in the field. It has also been suggested that this neuropeptide may be involved in the control of phase-related morphometric ratios of F/C and E/F (F=hind femur length; C=maximum head width; E=elytron length) as well as the development of antennal olfactory sensilla in the two locusts: injection of the peptide mimics crowding effects, thus inducing gregarious characteristics in solitarious (isolated-reared) locusts. Corazonin and related compounds are widespread among insects. Transplantation of the brain and corpora cardiaca from various donors to albino locusts indicates the presence of corazonin or corazonin-like substances in all 18 insect orders so far screened except for the Coleoptera.