The yolk sac blood islands have long been recognized as the first site for blood cell emergence during embryonic development. Much of our understanding of how blood islands form is derived from studies conducted in chick embryos. Similar processes are thought to be active during murine blood island development. But, how strong is the evidence that the process of blood island formation in the chick coincides with that documented in the mouse? This review will discuss some of the principle studies that led to two widely accepted definitions of blood islands and will examine more recent studies performed in murine embryos to determine whether either of these popular definitions can be directly applied.