As a part of the International Baccalaureate (IB) Diploma Programme (DP) students participate in the Theory of Knowledge (TOK) course. This research used a mixed methods design to investigate the perspective of TOK teachers in IB schools worldwide. To address the research questions, quantitative survey data were analyzed from 1,534 participants, and focus groups with 33 TOK teachers were conducted in Australia, The Netherlands, and the United States. Quantitative data were analyzed using descriptive statistics, t-tests, ANOVA, and linear trends analysis to identify group differences. Qualitative data were coded and analyzed using an inductive approach. Overwhelmingly, survey results indicated that teachers value the TOK course. Teachers ranked the main purposes of the TOK course as 1) to develop an awareness of how knowledge is constructed, critically examined, and renewed by individuals and communities and 2) to help students make connections among academic disciplines and among thoughts, feelings and actions. Teachers ranked the main benefits to students as 1) students better able to critically evaluate knowledge and 2) students better able to identify and reflect on personal assumptions. Teachers strongly agreed that teaching TOK has been a valuable professional development experience for them. Specifically, they indicated it enhanced their own critical thinking and developed their interdisciplinary understanding. Survey results and focus groups suggest the main challenges are 1) assessment, 2) time, and 3) administrative issues (scheduling and class size). Regarding implementation, approximately half of DP coordinators indicated that TOK implementation was different than other IB Diploma Programme aspects, specifically that teacher support and time were different.