This chapter emphasizes the many methods currently being employed to study brain networks related to attention. We seek to set current studies into a historical background of efforts to understand how the brain selects among stimuli and resolves competing responses. We examine attention as an organ system with networks of neural areas related to several major functions such as maintaining the alert state, orienting to sensory events and resolving conflict between responses. We consider the anatomy and circuitry of these networks and examine the role of genes and experience in their normal development and of various pathologies. Finally we examine how our current knowledge of the psychophysiology of attention illuminates traditional issues in cognition about how attention operates. The field of attention is one of the oldest in psychology. At the turn of the twentieth century Titchener (1909) called attention "the heart of the psychological enterprise." Attention is relatively easy to define subjectively as in the classical definition of William James who said: "Everyone knows what attention is. It is the taking possession of the mind in clear and vivid form of one our of what seem several simultaneous objects or trains of thought." (James, 1890, p. 403). However, this subjective definition does not provide hints that might lead to an understanding of mechanisms of attention that can illuminate its physical basis in terms of underlying physiological process nor clarify its normal development and pathologies. For these goals it is useful to think about attention as an organ system with its own anatomy and circuitry that develops in early life under the control of genes and experience. This will be the focus of our chapter. The modern history of attention as an organ can be started with the important studies of Maruzzi and Magoun (1949) on the reticular activating system. About the same time, Hebb (1949) called attention to the importance of networks of neural areas (cell assemblies and phase sequences) in building conscious representation of stimulus input (see Posner & Rothbart, 2004 for a review of Hebb's contribution). In the last fifty years there has been steady progress in the development of methods that allow us to probe the mechanisms of attention at a physiological level. It is the development of these methods and their use to probe attentional networks that seems most relevant to the current handbook. In this chapter we first trace history of the development of methods that allow study of attentional separately from other cognitive functions. We examine the methods used to link attention to underlying brain mechanisms including studies of lesioned patients, recording of electrical activity noninvasively in humans or by use of implanted electrodes and efforts to understand the genes related to attention. These include the use of microelectrodes in alert animals beginning in the 1970s and early studies of neuroimaging using hemodynamic methods starting in the 1980s. After our historical review, we examine current studies within cognitive psychology to get an idea of the functions of networks in vigilance, visual search, and cognitive control tasks. We then examine the anatomical networks that underlie these functions using whenever possible the combined methods that have developed over the last half century to explore neural networks. We then consider evidence of how genes and experience shape the development of attentional networks. At the end we return to some of the major questions in cognition that concern attention and review their current state.