Internship is a vital educational experience to help prepare students for their future careers. However, given the dramatic changes in the workplace, more extensive research is needed that examines students' internship experiences. The cognitive apprenticeship model of instruction focuses primarily on the development of expert skills during participation in a culture of expert practice. This study highlights whether the presence or absence of selected characteristics of this instructional model influenced changes or lack of change in two interns' performances over time. Qualitative research design guided the formulation of research questions, data collection, and analysis procedures. To provide a detailed, in-depth longitudinal description of the interns' experiences, only two interns were selected as participants, each at a different site--one site reflecting characteristics of the cognitive apprenticeship model, the other not. Interns were chosen based on similarities in gender, social skills, English language skills, level of educational media education, and previous internship experience. To uncover conditions of the two internship experiences that positively or negatively affected performance development, each intern was observed in the field for eight hours a week during their three-month internship, and interviews were conducted with the intern, her mentors and the staff. Interns' written mid-term and final reports were also analyzed. The data were examined using a code set for the criteria of expert performance in the major tasks of each internship that was created from a task analysis of the field log. The data units in each category were explored for the presence or absence of selected characteristics of the cognitive apprenticeship instruction model. Identifying patterns that emerged over time in each setting and similarities and differences in change or lack of change in performance helped specify the conditions under which change occurred. A comparative case analysis helped further illuminate the usefulness of the cognitive apprenticeship model in understanding the interns' internship experiences, particularly their levels of performance by the end of their internships. Two clear conclusions emerged: First, the presence or absence of the selected characteristics of the cognitive apprenticeship model of instruction--modeling, coaching, scaffolding and fading, and articulation--critically contributed to development or lack of development of expertise. Second, although the model was useful for understanding what happened, other factors also needed to be systematically considered: A high level of the intern's prior knowledge and experience and of intern personal agency, as well as a situated learning experience, positively affected the development of performance; moreover, constraints in the workplace seemed to negatively affect performance development.