The study investigates the molecular function of channelrhodopsin-2 (ChR2), a microbial-type rhodopsin from the green alga *Chlamydomonas reinhardtii*. ChR2 is shown to be a directly light-switched cation-selective ion channel, rapidly opening upon absorption of a photon and generating large permeability for monovalent and divalent cations. The channel desensitizes in continuous light, with recovery from desensitization accelerated by extracellular H+ and negative membrane potential, while closing is decelerated by intracellular H+. ChR2 is primarily expressed in *C. reinhardtii* under low-light conditions, suggesting its involvement in photoreception. The predicted seven-transmembrane α-helices of ChR2 are characteristic for G protein-coupled receptors but reflect a different motif for a cation-selective ion channel. The study also demonstrates that ChR2 can be used to depolarize small or large cells simply by illumination, making it a useful tool for manipulating intracellular Ca2+ concentration or membrane potential.The study investigates the molecular function of channelrhodopsin-2 (ChR2), a microbial-type rhodopsin from the green alga *Chlamydomonas reinhardtii*. ChR2 is shown to be a directly light-switched cation-selective ion channel, rapidly opening upon absorption of a photon and generating large permeability for monovalent and divalent cations. The channel desensitizes in continuous light, with recovery from desensitization accelerated by extracellular H+ and negative membrane potential, while closing is decelerated by intracellular H+. ChR2 is primarily expressed in *C. reinhardtii* under low-light conditions, suggesting its involvement in photoreception. The predicted seven-transmembrane α-helices of ChR2 are characteristic for G protein-coupled receptors but reflect a different motif for a cation-selective ion channel. The study also demonstrates that ChR2 can be used to depolarize small or large cells simply by illumination, making it a useful tool for manipulating intracellular Ca2+ concentration or membrane potential.