Several streams of research provide us information about the brains of bilinguals: direct localizationist studies, studies that focus on individual differences, and studies that focus on specific languages' structures. The localizationist approaches ask relatively direct questions about brain organization in bilinguals. We treat those in two sections here: the first, section II, on pre-imaging localizationist approaches, the second, section IV. on localizationist approaches within the past two decades that employ current brain-imaging techniques. The strictly localizationist approaches to understanding the bilingual brain are complemented by an interesting set of studies that we cover in section III. These focus on behavioral studies that identify individual-difference parameters, such as age of L2 acquisition, that, one can assume, reflect factors in brain organization in ways we cannot yet fully understand. Indeed, as we will demonstrate in section IV, the imaging studies incorporate several of these behavioral categories (most notably age of L2 acquisition and proficiency) in their designs as well. Thus it is not always easy to separate out how these localizationist and individual-difference factors interrelate, but we attempt to keep them somewhat separate in this paper, for purposes of exposition. In the following section of the paper, section V. we discuss the somewhat smaller set of studies of linguistic structures specific to the languages a bilingual speaks that, too, appear to engage differences in brain organization and/or processing, even if we do not know yet precisely what these are.