All vertebrate organisms from embryonic development to death require red blood cells. These cells develop from progenitor cells that arise from multiple embryonic/extraembryonic sites at multiple development ages. A subset of these progenitor cells ultimately colonize the bone marrow and generate all blood cells. The multiple waves of erythroid development are tightly regulated and well conserved among different mammaLian species from mice to men. Details of erythroid development and the mechanisms that regulate this process are becoming clearer because of numerous experimental studies using animal models, in vitro culture of isolated erythroid progenitors including from human embryo/fetus, and identification of naturally occurring mutations that disrupt this process. This article will focus on the origins of erythrocytes and the process of globin switching in which erythrocytes switch from expressing fetal hemoglobin to adult hemoglobin during fetal development.