We study the development of apps on competing platforms. We show that competition leads to commissions exceeding those maximizing consumer surplus (and, a fortiori, social welfare) whenever raising one commission reduces rivals' app bases. We relate this nding to economies of scope in app development and, to illustrate it, consider a setting in which some developers can port their apps at no cost: as their proportion increases, app development is progressively choked-o. Fostering platform competition or interoperability may therefore fail to produce the desired results. Within-platform app store competition, together with appropriate access conditions, may constitute a more promising avenue.