Attention has been studied from a variety of different theoretical and scientific perspectives ranging from the examination of cellular mechanisms such as the graded potentials of single neurons to the complex systems encompassing multiple brain regions underlying consciousness. While historically quite a diverse set of behavioral phenomena have been investigated under the rubric of attention and many different methodological approaches employed, there has been considerable convergence of concepts regarding the nature of attention and the processes that underlie it. In the first edition of this book, a theoretical framework was proposed that specified four core elements of attention, each comprised of interrelated component processes [1]. To a large extent, the elements of this model have proven to be consistent with those put forth in other models of attention over the past 2 decades. In this chapter, the elements of this framework are reviewed from the perspective of different scientific approaches to the study of attention, and evidence supporting their validity is discussed. This framework will be revisited and discussed in relationship to other models and theories of attention in Chap. 20.