The aim of the experiments in this paper was to explore the relationship between top-down and bottom-up processes in visual search. Employing behavioral techniques, we first consider the possible role of the magnocellular visual pathway in visual search, and find that visual search does not necessarily depend on processing by this visual sub-system. We next use functional imaging (positron emission tomography) to explore the effect of varying top-down strategy during visual search. Our findings indicate that the neural processes underlying visual search are distributed over an extensive network of brain regions, with varying roles for different parts of the network as the dynamics of top-down vs. bottom-up influences shift. The conjunction of bottom-up processing with top-down attentional suppression of an irrelevant singleton could account for activity found in right primary visual cortex (V1). The conjunction of bottom-up processing with top-down attentional set could explain activity noted in the right superior temporal gyrus/insular cortex. The left lateral cerebellum appears to play a role in attention, either in signaling popout or in switching attention repeatedly between multiple visual attributes. Loci in left parietal cortex (parietal operculum/superior temporal gyrus, parieto-occipital fissure and precuneus) are implicated in attention-demanding search for a target shape. Returning to behavioral experiments, we find that, when multiple feature singletons compete for attention, interference between them is strongest for features closely related to the distinguishing target feature. This competition appears to be feature-level rather than object-level, and is characterized by a varying degree of specificity for different features. Task complexity modulates interference effects, even for abrupt visual onsets, which are often considered to capture attention involuntarily. Overall, our observations converge on the conclusion that visual search is extremely flexible and subject to considerable specificity of top-down control, although such specificity is clearly not absolute.