Microbes surround us. However, most of the time we are unaware of them. Our epithelial surfaces, which provide the physical barrier separating us from the environment, are usually free of signs of a battle taking place. Over the past several years, we have come to realize that the epithelium is capable of mounting its own battery of defensive chemicals, which are more extensive, more potent, and more exquisitely regulated than initially appreciated. I am referring not to the constituents of fluids, such as tears, saliva, and sweat, that are secreted onto the surface of epithelia, but to antimicrobial peptides, the . . .