Our microbiomes dictate more about our bodies and our lives than we may like to admit. The influences range from the obvious, such as intestinal health, to the surprising, such as our moods. The science is anything but settled, though. After years of connecting the dots between microbes and human health, scientists have begun using microbes for medicine—but is it precise enough, and at what cost? And can we use that knowledge to alter our own microbiomes for the better, even if we aren’t sick?