To study macromolecular crowding (MC) in living cells, one needs a method to measure it. Several existing approaches to quantify MC address slightly different aspects of crowding. If we define MC through protein concentration, it can be measured by quantitative phase imaging coupled with volume determination; both can be realized on a standard bright-field microscope. Osmotic cell theory can help identify the essential factors that control MC. Nevertheless, there are still many gaps in our understanding of MC regulation and, in particular, of the interrelationship between MC and cell stress or damage. Experiments show that MC is subject to homeostatic control and returns to its resting values following various disturbances. Severe cell damage causes an accumulation of water and a decrease in MC; however, based on limited data, water accumulation is restricted to one area of the cell (necrotic bleb), while the rest of the cell remains at normal density. Similar heterogeneous water distribution is observed in vacuolated mammalian cells. Intermediate degrees of stress tend to produce dehydration and an increase in MC. Apoptotic shrinkage is one common example of stress-induced dehydration, but the effect may be more general. A hypothesis on its mechanism is proposed.