Flocculation, the clump forming property of yeast, has long been appreciated in breweries and utilized as an off-cost method to enable the reuse of yeast cells. Members of the flocculin protein family were identified as the adherent proteins on the cell surface responsible for flocculation, and their properties have been investigated. Crystal structures of the adhesion domain of flocculins revealed their unique mode of ligand binding where a calcium ion is located in the middle of the interface between flocculin and the interacting sugar. Here we describe the most commonly used flocculation assay. The method is simple and easy, yet it is the most direct and reliable assay to evaluate the flocculation cellular phenotype.