In the past two decades, the issue of out-of-field teaching (OFT) has concerned policy makers and researchers alike who see raising teachers' subject matter knowledge as the main policy lever to improve teacher quality. The study of OFT has emerged as one of the important subfields of teacher quality and teacher labour market research. Researchers in the United States have found widespread and high reliance on out-of-field teachers at the secondary level. They argue that out-of-field teachers are so great in number that the negative influence on student learning is spreading. Researchers have argued that OFT could undermine efforts to improve teacher quality through the professionalization of the teaching occupation. Outside the United States, studies have found that out-of-field assignments exist in several countries. However, OFT as a phenomenon remains understudied in the international comparative literature. We are unaware of the exact nature and spread of OFT internationally. This study examines OFT from an international comparative perspective using the newly published data from OECD. I focus on math and science teachers who teach in public schools in 21 countries. Three questions are examined in this study. First, is there cross-national variations in out-of-field teaching? Second, what is the distribution of out-of-field teachers across schools? Third, are there any differences in teaching practices and received professional support between out-of-field and in-field teachers? I find out-of-field teachers share certain similar attributes: young and inexperienced teachers with substantial educational attainment who work on short contract and part-time bases. Out-of-field teachers are disproportionally concentrated in rural, small, and low-SES schools. While out-of-field teachers do not differ from in-field teachers in several measures of teaching practices and time allocation, they were not given enough on-the-job development opportunities to improve their teaching skills. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]