Robert S. Mulliken, Nobel Laureate in chemistry, always had the intention to write a book about his field of research: molecular orbital theory. This is his scientific autobiography, edited posthumously by his former student Bernard J. Ransil and complemented with a memoir by Friedrich Hund, his scientific protagonist. Mulliken describes his career and gives an account of the contributions of his friends and colleagues at home and in Europe where he frequently travelled. And last but not least, he gives an accurate history of how the molecular orbital theory originated and how it evolved in an atmosphere of international exchange. The book is written in a particularly lively style, full of reminiscences and scientific facts, interwoven to produce an account of the Life of a Scientist.