In 1998, the Wisconsin-based biologist James Thomson announced that his research group had made a major breakthrough by successfully isolating human embryonic stem cells (hES cells). On this side of the Atlantic, too, there have been important developments, both scientific and regulatory. Most recently, in February 2002, the Human Fertilisation and Embryology Authority announced that, pursuant to the Human Fertilisation and Embryology (Research Purposes) Regulations 2001,1 it had issued the first licences in the United Kingdom for basic hES cell research to be undertaken. Why have these, and similar, announcements prompted a sense of great excitement and anticipation but also, and in just about equal measure, a sense of tremendous anxiety and deep concern? Attempting to answer such questions, a House of Lords Select Committee has recently published the most detailed review yet in the United Kingdom of the science and ethics of stem cell research and the regulatory issues to which it gives rise.2 First, why the excitement? Stem cells are special because they are a potential source of new cells for example, blood stem cells will replenish blood cells, neural stem cells will replenish brain cells, skin stem cells will replace skin, and so on. ES cells, however, are special because they are still in a so-called pluripotent state. As pluripotent stem cells, hES cells have the potential to develop into any one of the 200 or so human cell types and, in natural development, they will duly differentiate in this way. Those stem cells that have already developed to perform one or more specialised functions are called 'adult' stem cells (regardless of whether they are located in an adult person); and, whilst adult stem cells might have a number of possible differentiated functions and, thus, are multipotent the othodox view is that their function in the human body is relatively stable. In other words, whilst say blood stem cells might be able to regenerate blood cells, they are not thought to have the capacity to regenerate brain cells; and, similarly, neural stem cells are not thought to have the capacity to regenerate blood cells. Having isolated, purified and cultured hES cells, the next step is to seek to understand and simulate the mechanisms by which they differentiate to become specialised adult stem cells. The importance of such an advance in our knowledge