Although all cells in the adult mammalian body derive from the zygote, there are many intermediate steps which contribute to the complex process of tissue and organ development. Cells which derive from the inner cell mass, referred to as embryonic stem cells, form the basis of the structures and body systems necessary for the formation of a defined pattern of growth during embryological and foetal stages of development. The characteristics of stem cells which set them apart from other cells in the adult mammalian body include their ability to retain an undifferentiated state and also to undergo self‐renewal, producing identical daughter cells which retain these same characteristics (Table 8.1). The zygote is totipotent and has the ability to develop into an embryo including its foetal membranes. In the early embryo, totipotent cells differentiate, eventually producing cells with increasingly specialised functions. The term ‘differentiation’ describes a progressive process whereby cells and tissues develop specific structural and functional roles characterised by their specialised physiological or biochemical activities. This process is preceded by ‘commitment’ to a particular cell fate where the cell may not appear morphologically differentiated but its developmental fate is determined. The term ‘committed’ stem cell is also used to describe cells which have undergone differentiation but are restricted in terms of the cell lineage into which they can develop. This stage is followed by a labile phase termed ‘specification’ where the cell is capable of differentiating without further stimuli. At this stage, however, cell commitment is capable of being reversed. This is followed by ‘determination’, a stage at which the fate of the cell cannot normally be reversed, irrespective of external signalling. Stem cell nomenclature relates to the characteristics and origin of the cells being described. Accordingly, the terms ‘embryonic stem cells’ and ‘adult stem cells’ are applied to those cells which derive from the inner cell mass of the mammalian blastocyst in the former and those present in mature tissues or organs in the latter. Pluripotent stem cells are those which can develop into all the cell types of the embryo with the exception of the trophoblastic cells. The term ‘multipotent’ is used to describe stem cells which have the capacity to develop into limited subsets of cell types, such as white blood cells or epithelial cells. In contrast, unipotent stem cells are restricted to a single differentiation pathway. ‘Progenitor cells’ belong to a category of cells related to stem cells, but have a limited self‐renewal capability, are usually more differentiated than stem cells and have the ability to divide a limited number of times before differentiating into definitive cell types.