The primary visual cortex (V1) is arguably the most studied area in the mammalian cortex, and one of the very few for which we can say something sensible about the computations that it performs. V1 cells are selective for the position, shape, size, velocity, color, and eye of presentation of a visual stimulus. The mechanism of this selectivity as well as its rationale have recently begun to be understood, although some aspects still constitute an area of intense debate.