L2 learners’ target language use has been examined for various purposes, including assessment of the learners’ proficiency and examination of the process of language learning. That is, target language use in the real life tasks on the level as automatized as that of native speakers represents the goal of L2 learning. More importantly, however, target language use has been proposed as a process that provides the learning opportunities, particularly since 1980s by communicative approach to SLA. Until the learners can communicate effortlessly without being conscious of the form of the language, how learners’ attention and awareness are or should be directed to the language form and meaning has been an important issue among the recent SLA theories, summarized as the interface/non-interface debate. This paper discusses how different major SLA theories view learners’ attention and awareness of language form during language use and how it has been measured for both process and result research. Finally, a recent neurobiolobical SLA model is introduced in terms of its methodological and theoretical contribution to SLA research.