The study of long-run supply depends crucially on how the entry of new firms affects the prices of inputs. The simplest assumption one might make is that entry has no effect on these prices. Under this assumption, no matter how many firms enter or leave a market, every firm retains exactly the same set of cost curves with which it started. There are many important cases for which this constant input cost assumption may be unrealistic; we analyze these cases later. For the moment, however, we wish to examine the equilibrium conditions for this constant cost case.