This study demonstrates that activating fast-spiking (FS) interneurons in the barrel cortex induces gamma oscillations and controls sensory responses. Using optogenetic techniques, the researchers selectively activated FS interneurons and found that light-driven activation at various frequencies (8–200 Hz) selectively amplified gamma oscillations. In contrast, activation of pyramidal neurons only enhanced lower frequency oscillations, indicating a cell-type-specific double dissociation. The timing of sensory input relative to a gamma cycle determined the amplitude and precision of evoked responses. The study provides the first causal evidence that distinct network activity states can be induced in vivo by cell-type-specific activation. FS interneurons generate gamma oscillations through synchronous activity, which is stabilized by feedback from pyramidal neurons. The results support the fast-spiking-gamma hypothesis and show that FS activity is sufficient and specific for inducing gamma oscillations. Gamma oscillations are a resonant circuit property, and natural gamma oscillations depend on FS activity. The study also shows that gamma oscillations gate sensory processing, with FS-induced IPSPs restricting sensory transmission during its peak and allowing transmission after its decay, leading to a temporal sharpening of cortical sensory responses. These findings highlight the role of FS interneurons in cortical oscillations and provide a unique application of optogenetic engineering for studying discrete neuronal cell types under active network conditions.This study demonstrates that activating fast-spiking (FS) interneurons in the barrel cortex induces gamma oscillations and controls sensory responses. Using optogenetic techniques, the researchers selectively activated FS interneurons and found that light-driven activation at various frequencies (8–200 Hz) selectively amplified gamma oscillations. In contrast, activation of pyramidal neurons only enhanced lower frequency oscillations, indicating a cell-type-specific double dissociation. The timing of sensory input relative to a gamma cycle determined the amplitude and precision of evoked responses. The study provides the first causal evidence that distinct network activity states can be induced in vivo by cell-type-specific activation. FS interneurons generate gamma oscillations through synchronous activity, which is stabilized by feedback from pyramidal neurons. The results support the fast-spiking-gamma hypothesis and show that FS activity is sufficient and specific for inducing gamma oscillations. Gamma oscillations are a resonant circuit property, and natural gamma oscillations depend on FS activity. The study also shows that gamma oscillations gate sensory processing, with FS-induced IPSPs restricting sensory transmission during its peak and allowing transmission after its decay, leading to a temporal sharpening of cortical sensory responses. These findings highlight the role of FS interneurons in cortical oscillations and provide a unique application of optogenetic engineering for studying discrete neuronal cell types under active network conditions.