The human body contains over 200 different specialized cell types, such as muscle, bone or brain cells. These cells carry out specific functions within the body, necessary for the health of an organism. Injury, disease or ageing can lead to the loss of specialized cells from the body. In many cases, such loss is irreversible, meaning that the diseased or lost cells can no longer be replenished by healthy ones. Cell therapy aims to introduce new, healthy cells into a patient’s body, to replace the diseased or missing ones. A challenge for this type of therapy is having enough cells for transplantation into a patient. This is because specialized cells, such as brain cells, are difficult to obtain from the human body. Also, specialized cells typically have a limited ability to multiply, making it difficult to produce sufficient numbers of cells required for certain cell therapies. Some of these issues can be overcome through the use of stem cells. Stem cells are unspecialized cells that have the ability to develop into other functional cell types. Importantly, some types of stem cells can be grown outside of the human body, thus allowing the production of a large number of cells required for successful applications of cell therapy in medicine. Two main types of stem cells are being explored in the context of cell therapy: pluripotent stem cells and tissue-specific (also referred to as adult) stem cells.