Abstract Stem cells are defined as precursor cells that have the capacity to self-renew and to generate multiple mature cell types. Only after collecting and culturing tissues is it possible to classify cells according to this operational concept. This difficulty in identifying stem cells in situ, without any manipulation, limits the understanding of their true nature. This review aims at presenting, to health professionals interested in this area, an overview on the biology of embryonic and adult stem cells, and their therapeutic potential. Keywords: adult stem cells, biological characteristics, cell therapy, embryonic stem cells, human diseases Although the initial concept of stem cells is more than 100 years old,1 and much of its biology and therapeutic potential has been explored in the past three decades, we still know little about their true nature.