One recent fruitful development in the area of second language (L2) acquisition has been Task-Based Language Teaching (TBLT). Consequently, L2 researchers have paid particular attention to the role that the employment of tasks plays in the development of interlanguage competence. Of particular interest have been the factors that increase the difficulty with which L2 learners perform and process pedagogical tasks. The present paper is an attempt to present a review of factors that have been

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... to affect learners' task performance, along with studies that have provided empirical evidence as to how these factors increase the cognitive demands of learners' task performance. Understanding the functioning of these task performance factors are of paramount importance to language teachers who are to follow TBLT principles since such understanding can provide them with sequencing criteria that can guide the order in which they employ tasks in their everyday teaching efforts. Almost all studies of task difficulty factors have investigated the role of these factors in learners' task performance with respect to three aspects of linguistic performance; i.e., complexity, accuracy, and fluency. Skehan defines these three aspects of linguistic performance as the following:  Complexity: the elaboration or ambition of the language that is produced (1996b, p. 22)  Accuracy: a learner's capacity to handle whatever level of interlanguage complexity s/he has currently attained (1996a, p. 46)  Fluency: the learner's capacity to produce language in real time without undue pausing and hesitation (1996b, p. 22)