On Monday, May 14, 2012, Dr Claude Gagnon passed away after a long fight against Parkinson disease. Claude was one of the outstanding and most respected reproductive biologists worldwide. His fundamental contributions to our understanding of sperm biology will withstand the test of time, and his service to the scientific community will serve as a beacon for coming generations. Claude earned his PhD degree in biochemistry at the Université de Montréal in 1974 at a remarkably young age. He first went to Basel University, Switzerland, to do postdoctoral studies with Dr Hans Thoenen from 1974 to 1976, and then to the Laboratory of Clinical Sciences, National Institute of Mental Health, Bethesda, Maryland, with Dr Julius Axelrod, from 1976 to 1978. He was appointed as an assistant professor in the Département de Pharmacologie, Faculté de Médecine, Université Laval, in Québec City in 1978 and was promoted to associate professor in 1983. In 1984 Claude was recruited as an associate professor by the Urology Division of the Surgery Department of McGill University to build the Urology Research Laboratory at the Royal Victoria Hospital, and in 1993, under his leadership, this lab was extended to the Montreal General Hospital; both hospitals are part of the McGill University Health Centre. During the 20 years he served as director of these laboratories, Claude made numerous world-class contributions in different areas of reproductive sciences (andrology, surgery, and prostate cancer) in close collaboration with clinical urologists. For his outstanding contributions, Claude received numerous honors, including Scientist Awards in 1983 and 1988 from the Medical Research Council of Canada, the Young Investigator Award from the Club de Recherches Cliniques du Québec in 1987 (given to an independent investigator within the first 10 years of his academic career), Chercheur-Boursier de mérite exceptionnel (Exceptional Merit Scholarship) from the Fonds de la Recherche en Santé du Québec (1988–1993), and the Award of Excellence from the Canadian Fertility and Andrology Society in 2003. As a biochemist, Claude first made significant contributions to our understanding of the role of protein carboxymethylation in secretion, but during the past 3 decades his passion for research focused on sperm physiology: he wanted to understand the molecular mechanisms involved in sperm motility, flagellar movements (using human spermatozoa and Chlamydomonas models), and sperm fertilizing ability. In the 1990s, his lab pioneered the concept that reactive oxygen species (ROS) play a physiological role in sperm activation. This was a groundbreaking achievement because, at that time, ROS action was considered to be strictly detrimental to sperm function. Claude and his team dug deeper and deeper into the signaling pathways that enable the spermatozoon to fertilize an egg; his goal was to understand the causes of male infertility in order to find a treatment for that condition. Claude contributed to the operation of many societies and played a leadership role in the organization of many congresses. He was the president of the Canadian Fertility and Andrology Society (1996) and the organizer of the International Symposium on Spermatology (1998). For that particular event, he demonstrated remarkable talents as an organizer and fund-raiser to assure the great success of the scientific meeting. Claude also rendered major service to the American Society of Andrology, both as an elected member of Executive Council and chair of the Industrial Liaison Committee, and was a loyal member of the Society for the Study of Reproduction. Claude had a gift for finding excellent collaborators and keeping lasting relationships with them. He established fruitful joint research projects with scientists all over the world. He could always find the right people and obtain grants to support these activities. Numerous graduate students, postdoctoral fellows, and clinicians were trained under his supervision; many of them have gone on to establish themselves as independent investigators in the area of reproductive biology and fertility research. All of his trainees warmly remember Claude for his guidance and his ability to motivate their progress. Claude exemplified for them everyday life as a scientist and a researcher. Uppermost in the mind of those who worked with him are Claude's sharp intelligence, his moments of major enthusiasm, and his ability to analyze and discuss results, as well as his ingenuity in proposing “way-out” hypotheses. One of Claude's common sayings was, “Speculation is the only inexpensive thing in research.” There were also the extreme moments associated with grant applications and the euphoria of getting them—his lab was continuously funded from the time he opened his lab in 1978 until 2011. Claude had a love for life and a superb sense of humor; his friends and trainees share wonderful memories of going on fishing trips and of numerous parties (birthdays, Christmas, and any good news of accepted articles, prizes, and even sunny days). Claude was an outstanding scientist and worldwide leader in his field; he had a talent for visualizing things that others would not have considered important. He was a dedicated teacher and mentor. In addition to his mother, wife, sons, grandchildren, and family members, he will be sorely missed by everyone who had the privilege to work, train, or simply discuss science with him, because he had the ability to make people feel like friends.