Cells are highly sophisticated information processing devices, sensing diverse inputs Abstract:to make complex decisions. Yet despite increasingly elegant ways to measure pathway outputs, we largely lack control over the inputs delivered to the cell in space and time. I will describe how optogenetic techniques can overcome this challenge to control the exact combinations, dynamics, and spatial locations of pathway activity in live cells. Applied to mammalian growth factor signaling, they revealed the Ras/Erk module accurately transmits a huge range of steady-state and dynamic signals to downstream response programs. A light-based screen uncovered many of these dynamics-sensitive responses, including a cell-cell communication circuit acting through IL-6 family cytokines.