Plants have several kinds of photoreceptors, which regulate growth and development. Recent investigations using Arabidopsis thaliana revealed that the newly found blue light receptor phototropins mediate phototropism, chloroplast relocation, stomatal opening, rapid inhibition of hypocotyl elongation and leaf expansion (1,2). Several physiological studies suggest that one of the intermediates in phototropin signaling is cytosolic Ca2+. Studies using phototropin mutants have demonstrated that phototropins induce an increase in cytosolic Ca2+ concentration. However, the function of Ca2+ in the phototropin-mediated signaling process remains largely unknown. This review presents findings about phototropin-mediated calcium mobilization and the involvement of calcium in blue light-dependent plant responses.