A generation of research in cognitive psychology has given rise to many tasks that tap at various aspects of attention. It is now widely agreed that attention is not a single thing and that its measurement needs a strategy to study each of its various aspects. While there is no widely agreed taxonomy of attentional operations, there is an important distinction between functions of obtaining and maintaining the alert state (alerting network), orienting to sensory events (orienting network), and regulating thoughts and behaviors (executive network). Neuroimaging has confirmed that these functions involve separate but overlapping areas of brain activity. Neurochemical and genetic studies have also provided some distinctions between brain networks involved in attention. Alertness as a function of one important attentional network is emphasized and methods to activate phasic and tonic alerting are reviewed and individual or group differences in the efficiency of network operations are discussed.