Bacterial mechanosensitive (MS) channels play a significant role in protecting cells against hypoosmotic shock. Bacteria that have been diluted from high osmolarity medium into dilute solution are required to cope with sudden water influx associated with an osmotic imbalance equivalent to 10 to 14 atm. The cell wall is only poorly expansive and the cytoplasmic membrane even less so. Thus, swelling is not an option and the cell must rapidly eject solutes to diminish the osmotic gradient and thereby preserve structural integrity. This chapter describes cellular assays of MS channel function and their interpretation.