Cell adhesion makes an important contribution to the maintenance of tissue structure, the promotion of cell migration, and the transduction of information about the cell microenvironment across the plasma membrane. An ability to quantitate adhesion has proven to be extremely valuable for those researchers studying the molecular mechanisms underlying these processes. This article will outline in detail two standard assays used for quantitating the adhesion of cells to an immobilized substrate. First, an attachment assay, which employs a colorimetric detection of bound cells, and second, a spreading assay, which employs phase contrast microscopy to measure the flattening of adherent cells.