ViaLight™ Plus BioAssay Kits use bioluminescence catalysed by the firefly luciferase enzyme to measure intracellular ATP.  Since ATP is central to the metabolism of all cells, the intracellular concentration is very precisely regulated.  When a cell dies, the ATP levels drop instantly.  The combination of these two observations means that the number of viable cells in a culture can be very precisely measured by measuring the concentration of ATP in that culture.  When compared to control cultures, cell viability modifying treatments can be conveniently studied.  ATP values higher than controls indicate proliferation and cultures with ATP concentrations lower than controls denote cytotoxicity. Bioluminescent detection of ATP is very simple and extremely sensitive. 
A proprietary reagent containing luciferase and its substrates is simply added to the cell extract and the light emission is directly proportional to the ATP concentration in the sample.  Since bioluminescence can measure pico molar concentrations of ATP, very low cell numbers can be detected (down to 10 cells per microwell).