Hematopoietic stem cells (HSCs) posses the ability to maintain the blood system of an organism from birth to adulthood. The behavior of HSCs is modulated by its microenvironment. During development, HSCs acquire the instructions to self-renew and differentiate into all blood cell fates by passing through several developmental microenvironments. In this chapter, we discuss the signals and cell types that inform HSC decisions throughout ontogeny with a focus on HSC specification, mobilization, migration, and engraftment.