This thesis investigates the nature of optional preposition omission in L2 English extraction constructions known as null prepositions, e.g. *Who did you speak? (Klein, 1993b). Although null prepositions are found among learners with different LIs, their properties have not yet been fully understood, because many properties and possible sources have been left unexplored. To advance our understanding of null prepositions (and ultimately of L2 optionality), this study examined (a) null prepositions in different constructions (wh-questions, relative clauses, and passives), (b) the relation between null prepositions and the structure of L2 extraction constructions., ( c) the role of learners' LIs, and (d) task effects, by administering four experiments (an acceptability judgment task, an elicited imitation task, and two self-paced reading tasks) to French-speaking and Japanese-speaking learners of English as well as to native English controls. While the results of the four experiments corroborate the claim in existing studies that null prepositions are universal properties of the L2 grammars of learners from different LIs, it is also demonstrated that whether null prepositions are observed depends upon several factors such as the distinction between A- and A'-extractions, learners' Ll, and task differences. Specifically, the results of the four experiments consistently indicate that null prepositions in passives are particularly persistent for both L2 groups. By contrast, it is shown that null prepositions in wh-questions and relative clauses are subject to differences in learners' Ll and task. To explain these results, three sources for null prepositions are proposed: the difficulty in suppressing the Case feature from prepositions; covert pied-piping (Klein, 2001); influence from the syntax of Ll extraction constructions. It is also argued that these sources are not random but may be constrained by more general properties of L2 (null) prepositions: L2prepositions seem to be represented as Case markers without interpretative/semantic properties (Perpifian, 2010).