Optical nanosensors have been designed to utilise the sensitivity of fluorescence for making quantitative measurements in the intracellular environment, using devices that are small enough to be inserted into living cells with a minimum of physical perturbation. Advantages over widely used fluorescence dye based methods are observed because the nanosensor matrix imparts two key benefits; (1) protection of the sensing component from interfering species within the intracellular environment and (2) protection of the intracellular environment from any toxic effects of the sensing component. This Highlight article discusses the recent developments in nanosensor technology and investigates the use of more complex sensing schemes that will expand the range of analytes that can be detected and quantified.