This book is the first critical and analytical study of the American Phage Group--a small group of scientists who gathered around Max Delbrück, Salvador Luria, and Alfred Hershey between 1940 and 1960--and how this novel research program became the foundation of the field of molecular biology. These three young, charismatic, and iconoclastic scientists were convinced of the importance of bacterial viruses (bacteriophages) to the study of the gene and of heredity in general. Based on substantial archival research, numerous participant interviews collected over the past thirty years, and an intimate knowledge of the relevant scientific literature in the field, William C. Summers has written a fascinating new history of the American Phage Group. Rather than a linear narrative of progress by past heroes, this book emphasizes the diversity and historical contingencies in the group's development.