All sexually reproducing organisms produce primordial germ cells, a small population of cells that differentiate into gametes of either sex and carry to- potency, an ability to develop into an entire new organism. The study of germ cells has undergone enormous advances in recent years and has entered into an explosive phase of new discoveries with the introduction of transgenic te- nologies and nuclear cloning. Basic knowledge and techniques developed for lower vertebrate and invertebrate systems have facilitated the study of higher vertebrates, including humans. Many experiments that have first been performed on lower vertebrates provided the tools and strategies that could later be applied to other less readily available mammalian systems. The discovery of centrosomes in ascidians and sea urchin eggs now benefits studies of fertility and infertility in mammals. External in vitro fertilization, now a common technique in assisted fertilization, has only been possible as a result of numerous studies in lower systems in which external fertilization is natural. Egg activation, first explored in sea urchin and ascidian eggs, now benefits techniques designed to increase cl- ing efficiency in farm and domestic animals. Gene manipulations and molecular methods have added to the possibilities of producing live offspring with en- mous biomedical, ecological, and economic implications. The two volumes on germ cells combine techniques in a variety of d- ferent systems; we have selected those systems that have provided landmarks in advancing our knowledge on germ cells.