The research project reported in this chapter consists of three studies in which syntactic complexity, lexical variation and fluency appear as dependent variables. The independent variables are task complexity and proficiency level, as the three studies investigate the effect of task complexity on the written and oral performance of L2 learners of different levels of linguistic proficiency. Task complexity was defined according to Robinson's Triadic Componential Framework in terms of the number of elements to be dealt with (Robinson 2001a, b, 2003, 2005, 2007). Linguistic performance was assessed by means of both general and specific measures of syntactic complexity, lexical variation and accuracy. In the final section of the paper, the results of the three studies, which seem to contradict Robinson's predictions, are compared and discussed, both with respect to the Triadic Componential Framework and the proficiency measures used. Appended are: (1) Example of a complex task (L2 French): The Bed & Breakfast task; and (2) Example of a text written by a student of French L2. (Contains 6 tables and 4 footnotes.) [For complete volume, see ED539539.]