Flocking - the collective motion of large numbers of organisms or other self-propelled entities - exhibits a number of strange and baffling phenomena. Indeed, its very existence in two dimensions would appear to violate a fundamental theorem of statistical mechanics, while in any spatial dimension, flocks exhibit giant number fluctuations far in excess of those predicted by the "law of large numbers" of statistics. In this talk, I`ll show that all of this mysteries can be explained by a very general "hydrodynamic" theory of flocks, which summarizes the behavior of ALL flocks in much the same way that the well-known Navier-Stokes equations summarize the behavior of all simple fluids. I`ll then discuss the impressive agreement between this theory and both simulations, and recent experiments showing giant number fluctuations of actin near cell walls.