Catholic schools in the United States support the Church's mission to evangelize; in doing so, they have an impact on the future vitality of the Catholic Church. Catholic school cultures make the schools authentically Catholic and distinguish them from other educational institutions. The Church depends on administrators and teachers in the schools to establish and maintain strong mission-focused cultures. The majority of teachers in Catholic high schools are members of the laity (MacDonald and Schultz, 2017). Questions have been raised about the ability of lay teachers in Catholic schools to nurture strong Catholic cultures in the schools because teacher education and training typically does not address Catholic school mission or training in the ways that teachers promote the mission. Knowledge and understanding of mission guide the shaping of school culture (Peterson and Deal, 2009) and are key factors that impact teachers' ability to promote strong Catholic cultures in Catholic high schools. This study examined Catholic high school teachers' knowledge of their schools' mission statements, teacher's degree of understanding of mission, and the relationships of knowledge and understanding of mission to the strength of Catholic cultures in Catholic high schools. The theoretical framework for this study comes from research on Catholic school mission, Catholic school culture, and Catholic school teachers, and focuses on the critical role that teachers play in establishing and promoting Catholic school culture. The major variables in this study are teacher knowledge of mission statement, teacher understanding of mission, and manifestation of mission in Catholic high school culture. Data for this national study came from anonymous on-line surveys responded to by 415 teachers in Catholic high schools. Cluster sampling was used to include teachers from six geographic regions defined by the National Catholic Educational Association. The "Catholic High School Teachers' Manifestation of Mission Survey" used in this study was developed by the researcher. Descriptive statistics, multiple regression analyses and one-way between subject analysis of variance (ANOVA) procedures were used in the analysis of data. Pearson product-moment correlation coefficients were used to examine relationships among the major variables and regression analysis was used to determine the predictive relationships among variables. The results of these analyses revealed that there are significant positive correlations among all three major variables. The strongest relationships are between teacher knowledge of mission statement and the strength of manifestation of mission in school cultures. Positive relationships also exist between knowledge of mission statement and understanding of mission and between understanding of mission and manifestation of mission. Teachers' age, religious affiliation, vocation, amount of Catholic schooling, academic area of expertise, and number of years teaching in a Catholic high school were all predictor variables of at least one of the major variables. Teachers' gender and the governance model of the school in which they are teaching were not determined to be predictive of any major variable. Findings of this research study provide information about the current status of Catholic high school teachers' knowledge and understanding of school mission and level of manifestation of mission. Findings also provide information about the relationships between teachers' knowledge of the mission statement, understanding of the mission, and the strength of Catholic school culture in their schools that can contribute to the development of teacher preparation and mentoring programs. [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.]