After cell lysis, the most often used second step in a protein purification procedure is some sort of a rapid, bulk precipitation step. This is commonly accomplished by altering the solvent conditions and taking advantage of the changes in solubility of your protein of interest relative to those of many of the other proteins and macromolecules in a cell extract. This chapter will focus on the two most widely used precipitation methods: (1) ammonium sulfate precipitation and (2) polyethyleneimine (PEI) precipitation. These two methods work through entirely different principles, but each can achieve significant enrichment of target protein if optimized and applied carefully.