Processing theories (Clahsen, 1984; Pienemann, 1997) have argued that the production of subordinate clauses is the last stage in the development of the second language learner. However, these theories did not refer to the relation between proficiency level and the production of complex syntax. Thus, more knowledge is required about the development of frequency and accuracy of complex syntax in EFL learners, and the relation between this development and proficiency level. Moreover, both second language and first language research (i.e. Salaberry& Lopez-Ortega, 1998) have revealed the impact of the discourse genre and task on the production of subordinate clauses, without adequate investigation of expository discourse. Hence, this book provides an exploratory study on the development of complex syntax across three proficiency levels of EFL learners through oral expository presentations. A number of syntactic indicators of oral development are revealed with different degrees of sensitivity towards the development of fluency, accuracy, and complexity. This monograph should appeal to English instructors, linguists, and professionals in oral assessment and linguists.