In sexually reproducing organisms, germ cells give rise to gametes and serve as the link between generations. A key event during germ cell development is the decision to leave mitosis and enter meiosis, leading to the formation of mature eggs and sperm. Germ cell development involves unique gene expression programs and the establishment of epigenetic marks that ensure success during fertilization and embryo development. Written and edited by experts in the field, this collection from Cold Spring Harbor Perspectives in Biology reviews the biology of germ cells in metazoans. The contributors describe the undifferentiated state of germline stem cells, the triggers for meiotic entry, and the transcriptional and post transcriptional controls during spermatogenesis and oogenesis that lead to the formation of mature gametes. The expression of sex linked genes and the establishment of genomic imprinting in the germline are also covered. This volume, which includes discussions of gamete recognition proteins, egg activation, and genetic reprogramming following nuclear transfer, is an indispensable reference for cell, molecular, and developmental biologists and anyone wishing to understand the implications of germ cell biology for reproductive technologies.