In organisms, physiological noise arises from many sources—from changes in the external environment, from changes in internal physiological state, and from stochastic effects from the small numbers of particles interacting. I will discuss both destructive and constructive consequences of physiological noise, and I will propose a unified view for how organisms have evolved in response to that noise. The first big idea is that homeostasis evolved primarily to minimize noise in physiological channels. In effect, homeostasis provides quiet physiological backgrounds for the rapid, clear transmission of all kinds of physiological information. The second big idea is that stochastic events (noise) during development can diversify intra-organismal phenotypes in a way that makes them more robust to future environmental insults. I argue that this intra-organismal diversity—called mosaic physiology—may play roles as important in organismal biology as the roles already imagined for phenotypic plasticity.