BACKGROUND:The Patient-Centered Medical Home (PCMH) has become a dominant model of primary care re-design. The PCMH model is a departure from more traditional models of healthcare delivery and requires significant transformation to be realized. OBJECTIVE: To describe factors shaping mental models and practice culture driving the PCMH transformation process in a large multi-payer PCMH demonstration project. DESIGN: Individual interviews were conducted at 17 primary care practices in South Eastern Pennsylvania. PARTICIPANTS: A total of 118 individual interviews were conducted with clinicians (N=47), patient educators (N=4), office administrators (N=12), medical assistants (N=26), front office staff (N=7), nurses (N=4), care managers (N=11), social workers (N =4), and other stakeholders (N = 3). A multi-disciplinary research team used a grounded theory approach to develop the key constructs describing factors shaping successful practice transformation. KEY RESULTS: Three central themes emerged from the data related to changes in practice culture and mental models necessary for PCMH practice transformation: 1) shifting practice perspectives towards proactive, population-oriented care based in practice-patient partnerships; 2) creating a culture of self-examination; and 3) challenges to developing new roles within the practice through distribution of responsibilities and team-based care. The most tension in shifting the required mental models was displayed between clinician and medical assistant participants, revealing significant barriers towards moving away from clinician-centric care. CONCLUSIONS: Key factors driving the PCMH transformation process require shifting mental models at the individual level and culture change at the practice level. Transformation is based upon structural and process changes that support orientation of practice mental models towards perceptions of population health, selfassessment, and the development of shared decisionmaking. Staff buy-in to the new roles and responsibilities driving PCMH transformation was described as central to making sustainable change at the practice level; however, key barriers related to clinician autonomy appeared to interfere with the formation of teambased care.KEY WORDS: patient-centered care; primary health care; delivery of health care; workplace; program evaluation.