An important aspect of advanced academic writing is the ability to incorporate source material. Yet this ability often poses considerable challenges for international ESL students. Although a number of scholars have called for the explicit teaching of how to acknowledge and incorporate source materials, research evidence on whether explicit instruction is effective is relatively scarce. This paper reports on a classroom based study which investigated the efficacy of such instruction. Using ESL learners' performance on two integrated tasks (pre and post instruction), the study investigated whether learners' use of sources improves in response to instruction. The study found that instruction seems to lead to a reduced reliance on the language of the source materials. However such decreased reliance on the language of the sources may lead to greater inaccuracies in the attempted interpretation and incorporation of ideas from the source texts. The findings suggest that the ability to incorporate source materials may follow a gradual developmental path and this in turn has implications not only for instruction but also for the development of appropriate descriptors to assess the ability to incorporate sources. (Contains 2 tables and 5 examples.)