Introduction The essential claim of this chapter is that the concept of attention is necessary in order to understand virtually every aspect of second language acquisition (SLA), including the development of interlanguages (ILs) over time, variation within IL at particular points in time, the development of L2 influency, the role of individual differences such as motivation, aptitude and learning strategies in L2 learning, and the ways in which interaction, negotiation for meaning, and all forms of instruction contribute to language learning. The theoretical issues dealt with in this chapter are organized around the basic assumptions, definitions, and metaphors concerning attention in psychology and include the idea of attention as a limited capacity, the notion of selective attention, and the role of attention in action control, access to awareness, and learning. It turns out that, like most psychological concepts initially based on common experience, attention is not a unitary phenomenon, but refers to a variety of mechanisms. These include alertness, orientation, preconscious registration (detection without awareness), selection (detection with awareness within selective attention), facilitation, and inhibition. This does not diminish the centrality of attention, in its several manifestations, for learning. Although recent evidence, discussed towards the end of this chapter, indicates the possibility of some unattended learning, this appears limited in scope and relevance for SLA. There is no doubt that attended learning is far superior, and for all practical purposes, attention is necessary for all aspects of L2 learning.