Human skin, the body's largest organ, functions as a physical barrier to bar the entry of foreign pathogens, while concomitantly providing a home to myriad commensals. Over a human's life span, keratinized skin cells, immune cells, and microbes all interact to integrate the processes of maintaining skin's physical and immune barrier under homeostatic healthy conditions and also under multiple stresses, such as wounding or infection. In this Review, we explore the intricate interactions of microbes and immune cells on the skin surface and within associated appendages to regulate this orchestrated maturation in the context of both host physiological changes and environmental challenges. Copyright © 2014, American Association for the Advancement of Science.