Physical chemical parameters inside the cell are markedly different then in dilute solution, and hence need to be taken into account when explaining the biology of the cell. Herein I present the first sensors that allow quantification of the ionic strength and macromolecular crowding inside living cells. I will show how these sensors open up the possibility to obtain a better understanding of the intracellular crowded environment. The probes allow quantification with high spatiotemporal precision during stress, revealing how the conditions inside a cell change, and how cells may respond