This hugely influential book, published in 1966 as a 60th birthday tribute to Max Delbruck, is now republished in a 40th anniversary edition. In addition to the landmark collection of 35 essays by pioneers of molecular biology, this edition contains an additional essay by Sydney Brenner, one of the few influential molecular biologists of the time not to contribute to the original book. The essay contains material prepared for the original edition, but not submitted at the time, and a newly written view of the phage group seen from the perspective of someone close to, but not an intimate member of, that famous school.