Microtubules are dynamic tubulin polymers in the cytoskeleton that serve as conduits for cellular transport and scaffolds for cellular motility. Microtubules are also critical components of the mitotic spindle required for chromosomal segregation during cell division. Thus, inhibitors of microtubule assembly and dynamics, such as colchicine or paclitaxel, are potent mitotic poisons with antiproliferative properties. Because their low specificity can lead to systemic toxicity, improved approaches to targeting the delivery or bioactivity of these compounds could enhance their utility as probes of microtubule biology and their selectivity as targeted chemotherapeutics. Borowiak et al. now report a photopharmacological approach for the reversible small-molecule control of microtubule dynamics. Building on their earlier successes with light-induced gating of ion channels, the authors surmised that they could achieve light control of microtubule dynamics by replacing the central carbon-carbon bond of combretastatin A-4, a colchicine analog, with an azobenzene moiety that enables light-dependent isomerization between an active cis form, which assembles the tetramethoxyaryl pharmacophore, and an inactive trans form. They synthesized a series of these 'photostatins', and demonstrated that violet light induces the active cis conformation, whereas green light switches to the inactive trans conformation in a highly reversible and durable manner induced by low-intensity light. Furthermore, in the dark, the system decays to the more stable trans photostatin by providing an 'off'-state background of bioactivity. Biochemical assays demonstrated that cis-photostatins compete for the colchicine-binding site on tubulin and inhibit tubulin polymerization in vitro. Across several cell types, photostatins display 250-fold greater cytotoxicity under violet light illumination than in the dark, inducing mitotic arrest and apoptotic cell death. The photostatins were used to toggle microtubule dynamics inside living cells, to pause and reorient embryonic development in Caenorhabditis elegans, and to perturb the cytoskeleton in live mice by local tissue illumination. This unique system offers a useful tool for probing mechanistic questions in microtubule biology with spatial and temporal control, and also shows promise for tumor-specific chemotherapy, by localizing the cytotoxicity of antimitotic poisons within light-targeted tumor tissues.