Teacher leadership (TL) research has often failed to investigate equity and social justice as well as to consider national contexts and cultural differences. This study examines the relationship between TL and student achievement while investigating the moderating role of TL on the association between school poverty and student achievement in two countries (i.e., the United States and South Korea). TL is defined by two primary practices or activities: (a) supporting professional learning of peers (i.e., "collaboration," "cooperation") and (b) influencing school decisions (i.e., "distributed leadership," "decision-making responsibility"). I address three research questions: (a) How are TL practices reported by teachers and their school principals in the US and Korea? (b) How are TL practices associated with differences in student achievement in the US and Korea? (c) Do the associations between school poverty and student achievement vary according to TL practices in the US and Korea? Using 2015 PISA data, I utilized hierarchical linear modeling (HLM) to address the research questions and test specific hypotheses. The present study first found that the two TL practices (i.e., TL in peer learning and TL in school decisions) were weakly correlated to each other in both countries. Student achievement in the US was positively correlated with TL for school decisions (distributed leadership and decision-making responsibility), whereas student achievement in Korea was correlated to TL in peer learning (i.e., collaboration and cooperation). In addition, HLM analysis partially indicated that among the four different TL practices, only (budgeting- and curriculum-related) decision-making responsibility was statistically related to higher student achievement in the US. In Korea, collaboration (i.e., observing and providing feedback; joint activities) was negatively related to student achievement, while cooperation (i.e., exchanging teaching materials; ensuring common standards together; and attending team conferences). Furthermore, the results showed two significant patterns of the relationships among TL, student achievement, and school poverty. First, negative relationships between school poverty and student achievement were weaker in schools with a higher degree of TL (i.e., "distributed leadership" in the US, "curricular decision-making responsibility" in Korea). Second, the negative relationship between school poverty and student performance was great in schools with a higher degree of teacher "collaboration" in Korea. These findings have implications for theory and policy. First, these two major TL practices (i.e., TL in peer learning and TL in school decisions) that may be independent rather than interconnected concepts. Second, the conceptualization of TL should include the effects of educational systems and cultural contexts. Third, policymakers need to understand the different associations between TL practices and student achievement by country before formulating policy. Finally, a more valid and reliable database with elaborated measurement would enable future researchers to rigorously assess the relationships among TL, student achievement, and school poverty. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]