Abstract Gas exchange in developing fish is impeded by a series of resistances. Diffusive resistances dominate early in development; convective resistances become progressively more important as development proceeds. Body surfaces, the diffusive boundary layer (DBL), and, during embryonic development, the egg capsule and enclosed perivitelline fluid (PVF) are the major diffusive barriers. The relative impacts of these barriers change during the course of development as their physical structure is altered and/or the organism acquires the ability to actively ventilate the various fluid compartments (i.e., PVF and DBL). Gas entry into or exit from the body initially is entirely transcutaneous. A few species develop temporary specialized gas exchange structures as embryos, but the vast majority depends solely on the skin until the internal gills begin to form. However, the time of gill formation varies considerably among species, ranging from just before hatch to near the end of the larval period. This article examines the nature and relative impact of the various barriers to gas exchange and how these change during the course of development. Particular attention is focused on the transition from cutaneous to branchial respiration and the profound impact this has on the ability of the developing fish to satisfy expanding exchange requirements.