Background/Context Despite the tremendous amount of effort devoted by many mathematics educators to promote, defend, and implement reform-based mathematics education, procedural mathematics, which locates mathematical correctness in the procedures learned from textbooks and teachers, persists. Many researchers have identified school and classroom culture as the source of the problem; however, the exact meaning of school culture and its influence on teachers’ practices remains unclear. What is needed is a clearer understanding of classroom culture and how it influences practice. Purpose The purpose of our study was to examine how the aspects of a culture reinforce each other (and how they resist aspects alien to the cultural system) to understand the sui generis nature of culture. We use five aspects or indicators of culture—language usage, standard practices, tools and equipment usage, ongoing concerns and values, and recurring problems—to describe how they work together to create a culture. Population/Participants/Subjects The primary participant in this study was an eighth-grade mathematics teacher renowned for being a good teacher whose teaching conformed to the intentions of the reform-oriented National Council of Teachers of Mathematics (NCTM) standards, with a particular emphasis on problem solving. Research Design An ethnographic case study was conducted. Conclusions/Recommendations Although Ms. Bryans appropriated some of the rhetoric and practices of reform mathematics, her goals and assessment methods and most of her instructional methods were inconsistent. The analysis of the case shows that three conceptions of culture—individual, interactive, and collective—lead to quite different understandings of the problem. This case suggests the importance of differentiating each of these three conceptions of culture and discusses their implications for educational reform policies and professional development efforts.