One of the principal tasks of the nervous system is to generate internal representations of the world, in order that we might best interpret the present, predict the future, and thus pass our genes to the next generation. For this reason, it seems quite surprising that the nervous system is so noisy. This noise is reasonably well characterized at the level of ion channels and individual cells, but it is not clear why such a level of noise is tolerated. In this talk, I will describe some of the major sources of noise in the nervous system, and will discuss some of the current puzzles regarding how the effects of noise scale in the large-N limit.