Are you interested in steering life processes at a molecular level, telling enzymes when to stop and cells when to die? Would you like to understand techniques that tag, light up, and monitor structures at the limits of modern microscopy? Photobiochemistry – the application of light to the chemistry of living organisms – lends itself to both measurement and control. After reviewing a few of the ways molecules can interact with radiation to change their shapes and behaviors, we’ll dive into the current literature, discussing examples of photoactive substances that are relevant to basic biology and medicine. Principles from molecular orbital theory, physical organic chemistry and medicinal chemistry will be introduced through guided exploration of star examples that range from “cardiac switches” to anti-DNA warheads.