Transwell migration assays have been widely used for studying the motility of different types of cells including metastatic cancer cells. The assay is also useful in screens for compounds that act as chemoattractants or inhibitors of chemotaxis for cells. The assay employs a permeable layer of support, usually a tissue-culture-treated microporous membrane, which is positioned between two compartments that mimic two different sets of microenvironments for cell survival/growth. Cells on one side of the membrane, when sensing chemoattractants placed on the other side of the compartment that diffuses through the membrane, can migrate through the pores in the membrane towards the source of the chemoattractants. Cells that migrate across the membrane can be quantified by fixing and counting. Human breast epithelial adenocarcinoma MD-231 cells grow relatively fast and are metastatic. The MB-231 cell line is used here to describe the procedures of an in vitro cell migration assay using the transwell apparatus.