Stem cells are always regarded as cells with unique and extraordinary properties. SCs are able to self-renew and differentiate into specialized cells and this is a great advantage to maintain homeostasis in the body. The cells can divide with two different strategies and they are influenced by intrinsic and extrinsic factors of their microenvironment in which there are: the niche. The internal signals are represented by the genetic information of the cells, the external signals come instead from the microenvironment and they are physical or chemical signals. Stem cells are classified into embryonic stem cells and adult stem cells. Embryonic stem cells are derived from the inner cell mass of the blastocyst, these have great potential and over the years researchers have studied their properties and the importance of keeping them in appropriate culture conditions. Adult stem cells are found in a large number of tissues and have the very important role to replace damaged cells in living tissue. SCs can also be classified according to their potential, so they can be defined as totipotent, pluripotent, multipotent and they can differentiate respectively in a decreasing number of specialized cells of the body.