This thesis is concerned with the phenomenon known as overpassivization as it pertains to Korean ESL students. This thesis focuses particularly on the effect of animacy, subjecthood and conceptualizable agents in discourse on the rate of overpassivization. Other factors that are considered and analyzed are the effects of interlanguage interference, such as morphosyntactic variations of L1, and proficiency levels. The aim of this thesis is three fold. The first objective is to examine what specific difficulties with overpassivization exist particular to L1 Korean learners of L2 English. Animacy constraints and morphosyntactic interference are the primary focus of this line of inquiry. The second of these objectives is to identify how much of the learnability problems associated with the overpassivization phenomenon are L1 specific and L2 universal. This will be accomplished by comparing the results generated in the original research of this thesis concerning L1 Korean ESL learners with the results existent in the literature about the phenomenon with other L1 groups of L2 English. And finally, the goal of this thesis is to arrive at some pedagogical implications that will assist L1 Korean students overcome the learnability problems associated with overpassivization.Chapter 1 provides the theoretical background necessary for understanding the overpassivization problem. It defines the problem as it has been identified in the relevant literature and explains the unique properties of the unaccusative verb which is the primary source of the problem. Three existing theories, which seek to explain why there appears to be a universal learnability problem in L2 associated with the unaccusative verb, are explained along with the current state of research into the problem, as applied specifically to L1 Korean. Chapter 2 provides the details of the original research completed for this thesis. There are three separate experiments conducted with Korean ESL learners. The first was a translation exercise designed to provide insight into how animacy affects the learner’s choice of voice, active or passive, when translating from English to Korean. The second experiment was a forced choice task adapted from Ju (2000) who first identified the effect of a conceptualizable agent in discourse on the rate of overpassivization. This experiment was modified to see how animacy affects the saliency of the conceptualizable agent when manipulated in the subject position of the target sentence where students choose either an active or passive verb form. It was conducted with freshmen at Korean universities with an intermediate level of proficiency. The third experiment is also a forced choice task adapted from the original task looking for the effect of a conceptualizable agent, but in this instance, the conceptualizable agent, not the subject in the target sentence, was manipulated in terms of animacy to see if there was an observable effect. The results of experiment 2 and 3 are then analyzed together to see what effect proficiency level may have.Chapter 3 provides a summarized conclusion that addresses how the experimental results of this thesis relate to the competing theories, what issues specifically affect Koreans, and how the findings relate to previous research with other L1 groups. Based on the findings in chapter 2, the implications for pedagogy are briefly outlined before the chapter concludes with acknowledgement of the problem areas in this thesis and areas of future research.