Understanding the causal relationship between activity patterns in neural circuits and behavior requires the ability to perform rapid and targeted interventions in ongoing neuronal activity. I will describe a novel closed-loop all-optical strategy for dynamically controlling neuronal activity patterns in awake mice. This involves rapid tailoring and delivery of two-photon optogenetic stimulation based on readout of activity using simultaneous two-photon imaging of the same neural population. This closed-loop feedback control can be used to clamp spike rates at pre-defined levels, boost weak sensory-evoked responses, and activate network ensembles based on detected activity. By optically 'yoking together' neighboring neurons, it can also be used to induce long-term changes in network dynamics. This approach thus allows the rate and timing of activity patterns in neural! circuits to be flexibly manipulated 'on the fly' during behavior, enabling new approaches for probing the neural code.