This study forms a holistic, rich description of the way a professional development experience transformed teachers of mathematics. This professional development targeted teachers from six underachieving schools in a southern state who trained for a week in inquiry techniques of instruction. The method used to study the impact this experience had upon participants is a case study design in which three of the high school teachers were the subjects of analysis. Additional data were incorporated from project administrators and other teachers and assistants involved with the professional development experience. Data sources include highly structured questionnaires, observations, semi-structured interviews, documents, archival records and e-mails. Data were gathered before, during and after the experience to compare findings. Themes were developed from the data sources to assess the way that the professional development transformed the pedagogies and the beliefs of the subjects. Four rival conjectures on how this experience affected the subjects were drawn from the data and current literature. The levels of teacher employment of the themes vary and were dependent on the previous experience of the teachers, the amount of discussion on the themes, content and pedagogical knowledge and perceived professional needs of the subjects. Results suggest that designers of professional development take into account the previous experience of the participants, especially their perceived professional needs and beliefs. Designers also need to be aware of participant's content knowledge and pedagogical skills. The findings of this study impact the planning, execution and evaluation of professional development programs in education and other fields. The following are appended: (1) Texas Collaborative Observation Protocol; (2) Committee Member Observation Protocol; (3) Project Inquire Teacher Semi-Structured Interview Protocol; (4) Project Inquire Administrator Semi-Structured Interview Protocol; (5) Project Inquire Teacher Highly-Structured Questionnaire; and (6) Questionnaire Protocol for Dray after Inquire. (Contains 2 tables and 7 figures.)