As is the case in traditional second language (L2) acquisition research, a major question in the field of L2 real-time sentence processing is the extent to which L2 learners process the input like native speakers. Where differences are observed, the underlying causes could be the influence of the learner's first language and/or differences (fundamental or not) in the use of processing strategies between learners and native speakers. Another factor that may account for L1-L2 differences, perhaps in combination with others, is individual variability in general levels of proficiency or in learners' general cognitive capacities, such as working memory and processing speed. However, systematic research into the effects of such individual differences on L2 real-time sentence processing has yet to be done because researchers in the main attempt to control for individual differences in general cognitive capacities rather than to investigate them in their own right: nevertheless, a review of the current work on L2 sentence and discourse processing raises some interesting findings. An overview of this research is presented in this paper, highlighting what appear to be the circumstances under which individual differences in factors such as working memory capacity and proficiency do or do not affect L2 sentence processing. Taken together, the data suggest that it is only under certain experimental circumstances--specifically, when participants are asked to perform a metalinguistic task directing their attention to the manipulation at the same time as comprehending the input--that individual differences in such factors as insufficient L2 proficiency and/or cognitive processing limitations, like speed and working memory influence L2 learners' real-time processing of the target input. Under these circumstances, L2 learners of for instance, a higher working memory capacity or greater proficiency are more likely to process the input like native speakers. Otherwise, learners appear to shallow process the input, irrespective of individual variability.