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 fluency, 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.