This chapter reviews recent research that investigates second language performance from the perspective of sentence processing (on-line comprehension studies) and word recognition. It concentrates on describing methods that employ reaction time measures as correlates of processing difficulty or knowledge representation. This research suggests that second language learners employ much the same on-line processing strategies as native speakers, but that the L1 can also influence L2 processing. Reaction times in lexical decision experiments have been useful in exploring the relationship between the first and second language lexicons and automatic processes in lexical access. Finally, the chapter mentions some of the problems in this line of research, in particular the issue of individual differences in working memory and technological challenges.