The purpose of this qualitative case study was to identify and describe professors' beliefs about their subject matter knowledge in relation to their practice of learnercentered instruction. Professors' subject matter knowledge shapes course content at the university level, and professors practicing learner-centered instruction make an important contribution to undergraduate education. A description of professors' beliefs about their subject matter knowledge in relation to their instruction contributes to understanding exemplary practice. Recognized authorities in instructional development in higher education were given a set of criteria for learner-centered instruction, and identified a participant pool. Four participants teaching an undergraduate class in different departments at a research university in the United States, were selected. The theoretical framework guiding this inquiry was Polanyi's (1974) theory of personal knowledge. The theory was used to explain the individualized nature of the professors' subject matter knowledge, and the professors' passionate commitment to their teaching. Data were collected through conducting three semi-structured interviews, making classroom observations over a period of time, and examining instructional artifacts. Data were first analyzed within each case, and presented as individual portraits. The data were then recombined, and analyzed across the cases looking for common themes. The findings of the cross case analysis were presented according to the three questions guiding this study. Research validity and ethics were addressed through member checks with the participants and meetings with a peer reviewer. The inquiry found that the professors shared a belief that active knowledge is developed through a process of dialogue. Their belief about the process of dialogue infused their instruction. Although the professors did not use the term learner-centered instruction, they believed the focus of their instruction was to facilitate the students' process of integrating their prior knowledge with the course content. Each professor wanted the students to develop their own knowledge by discussing their understanding of the discipline's use of language and the discipline's way of thinking. The study's findings demonstrated the benefit of asking professors to articulate their beliefs about their practice. Study recommendations encourage instructional designers to expand their concept of working with professors to develop undergraduate instruction.