Louis (Lou) J. Guillette, Jr., one of the most influential research scientists in the field of environmental health, died 6 August 2015 at the age of 60. Lou had been battling non-Hodgkin’s lymphoma for almost a decade, but his passing, caused by bacterial pneumonia, came as a shock to everyone who knew him. He was an extremely energetic man with a passion for the natural environment. He revelled in being a scientist, adventurer, artist, and storyteller, and he was a talented communicator. Lou helped lead environmental research into a new era and is most recognized for identifying endocrine disruption in alligators and linking that groundbreaking research to consequences on human health. He was instrumental in instigating a paradigm shift in toxicology: the recognition that a change in hormone signaling during critical windows of development caused by low-dose exposure can lead to deleterious health effects, including cancer.