A cell-counting assay may determine the number of living cells, the number of dead cells, or the ratio of one cell type to another, such as enumerating and typing red versus different types of white blood cells. This is measured by different physical methods (light transmission, electric current change). But other methods use biochemical probing cell structure or physiology (stains). Another application is to monitor cell culture (assays of cell proliferation or cytotoxicity).
A cytotoxicity assay measures how toxic a chemical compound is to cells.
*MTT assay
*Cell Counting Kit-8 (WST-8 based cell viability assay)
*SRB (Sulforhodamine B) assay
*CellTiter-Glo® Luminescent Cell Viability Assay 
*Cell counting instruments and methods: CASY cell counting technology, Coulter counter, Electric cell-substrate impedance sensing
*Cell viability assays: resazurin method, ATP test, Ethidium homodimer assay (detect dead or dying cells), Bacteriological water analysis, Clonogenic assays, ...