The Problem: Employees sometimes behave contrary to the expectations and norms of the organization. These behaviors are termed "workplace deviance behaviors." In workplace deviance, employees consciously violate the rules of the organization, which, in turn, has the potential to negatively affect the organization itself, its members, or both. Workplace deviance, depending on the type of the behavior, is categorized into two dimensions: organizational deviance and deviance between individuals. Professionals working in successful schools carry out instructional activities; student, colleague, and parental relationships depend on certain norms. Teachers' deviant behaviors, by breaking school rules or interpersonal relationships, may harm the school and the students. Therefore, it is important to identify and prevent such deviant behaviors. Purpose: The aim of this research is to determine teachers' deviant behaviors according to the investigations carried out by education inspectors, and the level of effect these behaviors have on breaking down school rules and on interpersonal relationships from the perspectives of school principals. Methods: The mixed research method has been used in this research and the data have been collected in two stages, using first quantitative and then qualitative research methods. In the first stage, document examination, a qualitative research method, was used. The examined documents are the reports of investigations on teachers' deviant behaviors carried out by education inspectors during the 2008-2009 academic year. In the second stage, identified deviant behaviors among teachers were listed and a question form was developed. Using that form, 46 school principals were asked to evaluate how significant an effect the teachers' deviant behaviors have in breaking down school rules and in interpersonal relationships. Findings: After the document examination, it was determined that teachers repeated 24 types of deviant behaviors for a total of 131 incidents. Nearly all the deviant behaviors of teachers have a significant impact in flaunting rules or affecting interpersonal relationships within the school. Teachers' deviant behaviors have been divided into two categories: behaviors directed toward the organization and behaviors directed toward individuals. Conclusions and Recommendations: Results of this study, which aimed to identify teachers' deviant behaviors and the frequency of such behaviors, may not have recorded all deviant teacher behaviors. However, this is a pioneering study that identifies deviant behaviors in educational institutions. Qualitative research should be done to identify all deviant behaviors on the parts of teachers and the frequency of such behaviors. In addition, in order to be able to prevent teachers' deviant behaviors, studies, aimed at determining factors leading teachers to deviant behaviors should be carried out.