Development of the feto-placental blood vessels (human), or chorio-allantoic vasculature (mouse), is crucial for embryonic and fetal survival. While the processes governing embryonic vascular development are fairly well established, our understanding of feto-placental vascular formation is lagging decades behind. There are many unanswered questions in the field regarding potential progenitor populations, the timing of arterio-venous differentiation, the molecular cues that induce angiogenesis and the sources of these factors. In humans, particularly, there is little information on first-trimester placental vascular development or what pathologies may be caused by poor vascularisation. This Review discusses known processes of feto-placental blood vessel development in mice and humans, including their progenitors and derivatives (with their molecular markers), genetic knockouts and associated vascular phenotypes, trophoblast-endothelial signalling, co-occurrence with embryonic heart defects, genetic tools and imaging modalities targeting these vessels and pathologies that are impacted by vascular defects. Recent insight into early human placental vascularisation suggests it is more similar to the mouse than previously appreciated.