DEMANDS ON THE APPLICATION and flow properties of coatings have increased substantially, and because thickeners and rheology modifiers (TRMs collectively) play a key role in performance, they are among the most important components in a coatings system. Although TRMs are used in minor amounts in most formulations and are generally considered additives, aqueous coatings usually contain at least one, often two, and sometimes several to obtain the desired balance of properties. Due to present-day environmental concerns and the use of TRMs predominantly in aqueous coatings, the focus of this chapter is on water-borne, and especially latexbased systems. TRMs alsofind utility in solvent-borne coatings where they function primarily as flow modifiers and thixotropes. These products are discussed in later sections of this chapter.