Testicular germ cell tumours are a rare and bizarre diversion in the life cycle of the male germ line. These neoplasms are thought to originate during embryonic life from primordial germ cells (PGCs) which fail to undergo maturation into prospermatogonia. Maturation arrest and the development of aneuploidy in the PGC give rise to the precursor of germ cell malignancy, the testicular carcinoma in situ (CIS) cell. Thereafter, a complex series of genetic changes, coupled with the onset of puberty, can either convert the CIS cell into a malignant tumour made up of cells resembling primordial germ cells (a seminoma), or drive it down a pathway akin to parthenogenesis, so that it acquires a special property shared with cells of the early embryo – pluripotentiality, or the ability to differentiate into a wide range of somatic cells. The latter form of germ cell tumour, a teratocarcinoma, will contain primitive undifferentiated stem cells and multiple somatic tissues representing derivatives of all three germ layers plus the extraembryonic membranes which support development. Pluripotentiality is a property common to the oocyte, the cells of the early embryo up to the stage just after implantation, primordial germ cells, and the stem cells of germ cell tumours (Figure 1).