This chapter will outline in detail the two standard assays used in the author's laboratory for quantitating the adhesion of cells to an immobilized substrate. The attachment assay, which employs a colorimetric detection of bound cells, is based on Kueng et al. (Anal Biochem 182:16-19, 1989), and the spreading assay, which employs phase contrast microscopy to measure the flattening of adherent cells, is based on the method of Yamada and Kennedy (J Cell Biol 99:29-36, 1984).It is important to realize that cell adhesion is a complex process that involves many different molecular interactions, including receptor-ligand binding, changes in the fluxes through intracellular signaling pathways, and modulation of cytoskeletal assembly. Consequently, adhesion assays not only measure the contacts between a cell and extracellular adhesion proteins, but also provide information about other cellular events. For this reason, care needs to be taken before choosing to perform adhesion assays. The most common uses of adhesion assays are (a) to test the ability of a specific type of cell or cell line to adhere to a specific adhesive substrate, and (b) to test the sensitivity of a specific cell-substrate interaction to inhibitors, but it is also apparent that adhesion assays can be used to probe the contribution of other cellular processes.