The visual cortex is widely held to encode the attributes of visual stimuli through the activity of neuronal populations. How does this activity evolve over time? We stained the primary visual cortex of anesthetized cats with voltage-sensitive dye RH-1692, and imaged the dye fluorescence (Grinvald & Hildesheim, 2004). We stimulated with sequences of gratings flashed for 30-40 ms at random orientations and spatial phases (Ringach et al., 1997). We expressed population responses to this stimulus as a function of preferred orientation. We could predict these responses to a considerable degree on the basis of a simple linear filter that describes the average response to a given orientation as a function of time. This filter was separable, i.e. it was the product of a function of preferred orientation and a function of time. Separability indicates that activity in the orientation domain is a standing wave (it neither broadens nor sharpens over time). We then studied how activity evolves in the spatial domain. We stimulated with small patches of standing gratings whose contrast reversed at ~ 5 Hz. These stimuli elicited strong responses at twice that frequency (~10 Hz), consistent with an origin in complex cells. The activity extended over a few mm of cortex, with response phase showing a clear dependence on distance from the stimulated site. This behavior is evidence for a traveling wave. We conclude that visual stimuli elicit waves of activation in visual cortex: standing waves in the orientation domain, and traveling waves in the spatial domain.