The word “hormone” often popularly connotes teenagers and the changes that occur to their bodies as they grow and mature. Hormones, as we will see, do play a key role in animal growth and development—but that is only one of their roles. Hormones play diverse physiological roles in the body, in particular regulating an organism’s response to the environment and helping to maintain stable physiological conditions within cells or within the animal as a whole. We start by highlighting these functions before addressing the molecular mechanisms of how hormones work in section 38.2.