Support is requested for a Keystone Symposia meeting entitled Optogenetics, organized by Edward S. Boyden, Klaus M. Hahn and Chandra Tucker. The meeting will be held in Denver, Colorado from March 12- 16, 2015. The complexity of biological systems demands tools that enable temporally and spatially precise control of defined signaling pathways. Optogenetics is a new and rapidly evolving field that combines light control with genetic engineering, allowing conditional regulation of signaling processes with seconds and microns precision. Using optogenetic probes, the functions of specific molecules and pathways, the behaviors of individual cells, and the interactions of groups of cells in complex organisms can be precisely interrogated. This Keystone Symposia meeting seeks to bring together a broad range of researchers developing and using optogenetic probes with those developing related technologies. Specific goals include (i) bringing together scientists from a wide range of fields with relevant interests in optogenetics, (ii) providing an interactive forum for new investigators and highlight their research, and (iii) discussing the latest tools, hardware, and associated technologies and their application to scientifically or clinically relevan questions. We envision this meeting will facilitate the development and dissemination of a diversity of new optical tools, bring together a diverse but synergistic group of innovators and experimentalists, and ultimately open up new experimental avenues for study that will transform diverse fields.