The beginning of life may be a miracle to some, and a mystery to others, but it is certainly one of the most exciting and perhaps controversial fields of scientific investigation in the 21st century. Among the metazoa, life begins when an egg is fertilized by a sperm. The sperm provides a genetic blueprint from the father and perhaps some critical proteins. The egg provides a genetic blueprint from the mother together with a large reservoir of mRNAs and proteins that are required for DNA replication, cell division and the onset of zygotic gene expression. All of the thousands of genes in these two mature gametes are transcriptionally silent and remain so until fertilization.This work focuses on three biological systems, providing the reader with a clear understanding of the current state of affairs, and the ability to identify common principles as well as critical differences that are responsible for beginning the process of animal development. The essays presented will be of practical value to all those who are interested in improving fertilization in vitro, in designing novel methods of contraception, in developing preimplantation genetic diagnosis for various diseases, in cloning animals by transplanting nuclei from adult cells to an enucleated egg, and in the application of embryonic stem cells to curing genetic diseases or replacing damaged tissues. But above all, this volume is offered to those who simply have an insatiable curiosity about life and its beginnings.