Stem cells constitute a set of undifferentiated cells with the capacity to differentiate into other cell types and to self-renew. Stem cells can be: (i) totipotent, such as the ones of the zygote, which can give rise to any cell in an organism and to extraembryonic structures; (ii) pluripotent, such as embryonic cells that can differentiate into any cell in the germ layers; (iii) multipotent if they can differentiate into different cell types within specific lineages, such as hematopoietic stem cells; (iv) oligopotent, with the capacity to differentiate into only some cell types; and (v) unipotent, which gives rise to a specific cell type. In adults, there are multipotent, oligopotent and unipotent stem cells, being distributed throughout the organism. Their functions involve maintaining tissue homeostasis and regeneration.