In the classic Stroop effect, naming the color of an incompatible color word (e.g. the word RED printed in green ink; say, 'green') is much slower and more error-prone than is naming the color of a control item (e.g. XXX or CAT printed in green; say 'green'). This seemingly simple interference phenomenon has long provided a fertile testing ground for theories of the cognitive and neural components of selective attention. We present a sketch of the behavioral phenomenon, focusing on the idea that the relative automaticity of the two dimensions determines the direction and the degree of interdimensional interference between them. We then present an outline of current parallel processing explanations that instantiate this automaticity account, and we show how existing interference data are captured by such models. We also consider how Stroop facilitation (faster response of 'red' to RED printed in red) can be understood. Along the way, we describe research on two tasks that have emerged from the basic Stroop phenomenon - negative priming and the emotional Stroop task. Finally, we provide a survey of brain imaging research, highlighting the possible roles of the anterior cingulate in maintaining attentional set and in processing conflict or competition situations.