The peroxisomal membrane protein Pex3 and its cytosolic interaction partner Pex19 have been implicated in peroxisomal membrane biogenesis. Although these peroxins have been extensively studied, no consensus has been reached yet on how they operate. Here, we discuss two major models of their function, namely, in direct insertion of proteins into the peroxisomal membrane or in formation of peroxisomal membrane protein-containing vesicles from the endoplasmic reticulum. Pex3 can also recruit other proteins to the peroxisomal membrane (e.g. Inp1, Atg30, Atg36), thereby fulfilling roles in other processes such as autophagy and organelle retention. Recent studies indicate that Pex3 and Pex19 can also facilitate sorting of certain membrane proteins to other cellular organelles, including the endoplasmic reticulum, lipid droplets and mitochondria. This article is protected by copyright. All rights reserved.