The Wnt signalling pathway is clearly required for gastrulation in mammalian embryos, but little is known about its extra-embryonic and preimplantation functions. Here (p. 2961), Janet Rossant and co-workers investigate the requirements for Wnt signalling in early mouse development using a mouse line that carries a floxed allele for the porcupine homolog (Porcn) gene. Porcn is required for the acylation and secretion of all 19 mammalian Wnt ligands, so Porcn function represents a bottleneck for Wnt signalling. Using zygotic, oocyte-specific and visceral endoderm-specific deletions of Porcn, the researchers show that Porcn-dependent Wnt signalling is not required for preimplantation development or for implantation itself, and they confirm that gastrulation is the first Porcn/Wnt-dependent event in embryonic tissues. They also identify chorio-allantoic fusion as the first major Porcn/Wnt-dependent event in extra-embryonic tissues. Together, these findings show that, although Porcn-dependent Wnt signalling is important for embryonic and placental function, it does not have an essential role in preimplantation development or in blastocyst lineage specification.