Attention allows us to select the most important information at each moment in time and to enhance and differentially process that information while ignoring other, irrelevant information. This capacity is essential to nearly all cognitive processes. But, how does attention work at the level of cells and circuits? We are addressing this question by studying circuits that contribute to spatial attention in birds. We have identified a midbrain network that includes structurally specialized cholinergic, GABAergic and glutamatergic circuits, which perform many of the fundamental computations underlying attention, including filtering for stimulus salience, competitive selection of the most salient stimulus, and top-down enhancement of the quality of information. This network interacts extensively with the fronto-parietal forebrain network and is highly conserved across vertebrate evolution. We study the properties of these specialized circuits in chickens and owls, in behaving animals engaged in attention-demanding tasks, in vivo and in brain slice preparations. I will discuss our current understanding of how the computations performed by these circuits contribute to the control of attention.