Gamma rhythm ranges from 30 to 80 Hz, widely existing in neocortex. The key condition for the generation of gamma rhythm is the participation of inhibitory interneuron network. Spontaneous gamma rhythm is mainly related to parvalbumin inhibitory interneurons. In mouse, cat and monkey V1, the induced gamma rhythm is mainly derived from laminar 2/3 and 4B, and are well tuned by grating parameters. In primary visual pathway of cat and mouse, the high frequency gamma rhythm induced by brightness is derived from retina. However, brightness evoked gamma oscillation in monkey was observed in V1, but was not recorded in LGN. Dendrite-targeting somatostatin interneurons are critical for a visually induced, context-dependent low gamma rhythm (20-40 Hz) in primary visual cortex. The visually induced high frequency gamma rhythm (65-80 Hz) was mainly associated with PV cells. Increases in background light intensity trigger proportional increases in narrowband gamma oscillations (55-65 Hz) of mouse early visual pathway, which originated from ipRGCs cells in the retina. The gamma rhythm is variable in different physiological state, development stage and brain disease, implying the change in the visual information processing.