The measurement of forces between lipid membranes, DNA molecules, polysaccharides, and proteins is usually made by putting them under the osmotic stress of an excluded polymer that creates the arrays observed by x-ray diffraction. This procedure requires careful measurement of polymer osmotic pressures over wide ranges of molecular weight and concentration. While building up this large data set, we realized that there was material here to re-examine the theory of polymers under crowded conditions. A simple description and theory has emerged. At low concentrations, polymers follow the van't Hoff law for pressures linear in concentration. At very high concentrations, polymers follow the less familiar des Cloiseaux law for pressure going as the 9/4ths power of concentration. The big surprise is that these two limiting laws, combined with only one fitting parameter, suffice to describe a huge set of osmotic pressure data for any given polymer species for all measured molecular weights. This osmotic pressure of crowding is the driving force for transport of large polymers through relatively small pores.