The study of germ cells has undergone enormous advances in recent years and has entered into an explosive phase of new discoveries with the introd- tion of transgenic technologies and nuclear cloning. Basic knowledge and te- niques 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 st- ies of fertility and infertility in mammals including humans. 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 asc- ian eggs, now benefits cloning efficiency in farm and domestic animals. Gene manipulations and molecular methods have added to the possibilities of p- ducing live offspring with enormous biomedical, ecological, and economic implications. All sexually reproducing organisms produce primordial germ cells, a small population of cells that differentiate into gametes of either sex that carry to- potency, an ability to develop into an entire new organism. The two volumes on germ cells combine techniques in a variety of different systems and have selected those systems that have provided landmarks in advancing our kno- edge on germ cells.