Molecular switching is an interdisciplinary area of research that has its main emphasis at the interfaces between chemistry and materials sciences. The emphasis in molecular photoswitching has typically been on designing systems that can be used to deliberately trigger molecules to undergo structural and functional changes on command, providing light-induced control over a wide range of optoelectronic properties such as absorption, emission, refractive index, magnetism and conduction. It is clear that the area has significantly influenced how we think when we look to the next generation of molecular electronics, optical recording and display materials. A less developed use for molecular photoswitches is in how the changes in their electronic and steric structures can regulate chemical and biochemical reactivity, which is surprising considering the impact that this development would have on chemical synthesis, biology and medicine. This talk will illustrate the reversible regulation of the way molecules react or interact with other chemicals and complex biological systems based on structural changes. Examples will include systems based on the well-known dithienylethene architecture that unmask or release small molecules using light, behave as reporters in detection applications, and act as reversible co-factors and inhibitors of enzymatic systems. The changes in the optical properties of the molecules will be emphasized as they can be used to report the success of the photochemical and resulting chemical events. The use of nanoparticles as optical reporters and delivery systems will also be discussed.