The patient-centered medical home (PCMH) is four things: 1) the fundamental tenets of primary care: first contact access, comprehensiveness, integration/coordination, and relationships involving sustained partnership; 2) new ways of organizing practice; 3) development of practices' internal capabilities, and 4) related health care system and reimbursement changes. All of these are focused on improving the health of whole people, families, communities and populations, and on increasing the value of healthcare. The value of the fundamental tenets of primary care is well established. This value includes higher health care quality, better whole-person and population health, lower cost and reduced inequalities compared to healthcare systems not based on primary care. The needed practice organizational and health care system change aspects of the PCMH are still evolving in highly related ways. The PCMH will continue to evolve as evidence comes in from hundreds of demonstrations and experiments ongoing around the country, and as the local and larger healthcare systems change. Measuring the PCMH involves the following: Giving primacy to the core tenets of primary care. Assessing practice and system changes that are hypothesized to provide added value Assessing development of practices' core processes and adaptive reserve. Assessing integration with more functional healthcare system and community resources. Evaluating the potential for unintended negative consequences from valuing the more easily measured instrumental features of the PCMH over the fundamental relationship and whole system aspects. Recognizing that since a fundamental benefit of primary care is its adaptability to diverse people, populations and systems, functional PCMHs will look different in different settings. Efforts to transform practice to patient-centered medical homes must recognize, assess and value the fundamental features of primary care that provide personalized, equitable health care and foster individual and population health.