How and why animals behave the way they do has fascinated humans for centuries. Many insights into the causes and consequences of animal behavior have been gleaned by considering how an animal’s genotype and physical or social (e.g., competitors, mates, predators) environment influence behavior. Now, new studies are revealing that the trillions of microbes in and on most animals’ bodies also play a fundamental role in determining behavior. Microbes in the gut can influence levels of anxiety and depression in mice; blood-feeding mosquitoes select victims based on the microbial composition of human skin; and even the mating preferences of fruit flies are strongly shaped by microbes. Studies spanning laboratory animal models and natural field systems are essential for uncovering the nature and implications of these host-microbe interactions. This session explores how research across a diversity of settings is elucidating the mechanisms by which microbes alter behavior and vice versa, and the broader consequences of these effects. The session also highlights how understanding connections between microbes and animal behavior may translate into novel approaches for treating behavioral disorders in humans.