Experts have regarded continuity of care as a major component of good medical care for more than seven decades (Lee & Jones, 1933; Saultz, 2000). Simply put, the underlying assumption is that continuity facilitates interaction and communication between the patient and his or her physician, which helps develop mutual trust, stimulates the flow of information between them, bolsters the accuracy of the physician’s diagnoses, enhances selection of the most effective treatments, and increases patient compliance and adherence (Breslau & Haug, 1977). An extensive, recent literature review (Saultz, 2003) underscored the durability of continuity of care as a key concept. And a recent multinational study reaffirmed its importance to primary care physicians (Stokes et al., 2005). It is not surprising, then, that in 1996 the Institute of Medicine (IOM) identified continuity of care as a core attribute of primary care because it should result in better quality care and disease management, especially for older adults with one or more chronic conditions. More recently, however, the IOM elevated continuity of care to the status of a primary aim in its comprehensive call for national action to transform health care quality. Specifically, that aim is To establish and support a continuous healing relationship, enabled by an integrated clinical environment and characterized by the proactive delivery of evidence-based care and follow-up. (IOM, 2003, p. 49) In that same paragraph, the IOM explicitly recognized Shortell, Gillies, and Anderson’s (2000, p. 129) definition of clinical integration as … the extent to which patient care services are coordinated across people, functions, activities, and sites over time so as to maximize the value of services delivered to patients. This is not just an IOM or even an American point of view. Writing in the British Medical Journal, Haggerty and colleagues (2003, p. 1219) noted that “policy reports and charters worldwide urge a concerted effort to enhance continuity.” But what does continuity of care really mean? The answer to that question is not at all straightforward. To be sure, scholars have discussed and debated the conceptualization of continuity of care for decades (Christakis et al., 2004; Dietrich & Marton, 1982; Ettner, 1996, 1999; Haggerty et al., 2003; Hennen, 1975; Lambrew, DeFriese, Carey, Ricketts, & Biddle, 1996; Mainous, Baker, Love, Pereira Gray, & Gill, 2001; Mainous & Gill, 1998; Rogers & Curtis, 1980; Saultz, 2000, 2003; Saultz & Lochner, 2005; Shortell, 1977; Starfield, 1980; Steinwachs, 1979; Weiss & Blustein, 1996). Consensus has emerged, however, only on the multiple benefits that should accrue from continuity and on the multidimensionality of the concept itself. The expected benefits include improved doctor–patient relationships; enhanced physician knowledge of the patient; greater rapport and disclosure; increased compliance; reduced hospitalization rates; increased patient and physician satisfaction; reduced disability levels, costs, and number of missed appointments; and improved problem recognition and management (Rogers & Curtis, 1980). The major dimensions of continuity of care include informational, longitudinal, interpersonal, geographic, team-based, and familial (Saultz, 2003). This, however, is where the consensus over continuity of care ends (Haggerty et al., 2003). Indeed, in his extensive review Saultz (2003) identified no fewer than 21 instruments designed to measure continuity of care, several of which he noted had never actually been used. Saultz placed these instruments into three categories: (a) those that do not require an assigned (i.e., primary) provider, (b) those that do require an assigned provider, and (c) those that measure continuity of care at the family level. In general, measurement instruments in the first category rely on concentration statistics, including Gini coefficients (Shortell, 1977; Steinwachs, 1979), Herfindahl indices (Eriksson & Mattsson, 1983; Smedby, Eklund, Eriksson, & Smedby, 1986), and related fractionals (Roos, Roos, Gilbert, & Nicol, 1980). These are designed for use with administrative data, although they can also be used with self-report or survey data that provide the number of visits to each of the various physicians seen during the target interval. The major drawback of these instruments is that, by focusing on the largest fractional, they ignore the medical specialty of the physician and assume that the physician seen most often is, in fact, the primary (in the sense of usual) physician. The next category of continuity of care measures is remarkably similar, except that these measures require the identification of a primary physician. Various themes within this category address fractionals among the identified provider (Breslau & Haug, 1977; Smith, 1995), the duration of the relationship with the usual physician (Weiss & Blustein, 1996), and the rate of provider turnover (Merenstein et al., 2001). Another subset within this category involves patient identification of the usual physician, which requires moving beyond sole reliance on administrative data (Ettner, 1996, 1999). Although these measures do require the identification of a usual provider, they generally do not impose restrictions on who that usual provider can be or how long the patient–practitioner relationship should be in existence, and their algorithms are not theoretically motivated. The last category of continuity of care measures is specific to family practice. These instruments focus on the extent to which the family unit receives continuity of care from the same family physician. Such measures focus either on a global index (Murata, 1993) or on the average continuity of care in the family with the same provider (Godkin & Rice, 1984). As interesting as these family-based measures may be, they have little applicability outside of family practice, especially for older adults. This brief review should make two things clear. First, continuity of care is presumably very important, especially for older adults. Second, no theoretically driven measure exists for the standard assessment of continuity of care. This is especially problematic given the increasing availability and use of administrative data for evaluating the quality of health care and disease management (IOM, 2003). Accordingly, we report here on the conceptualization, operationalization, and initial validation of an interpersonal continuity of care measure for use with Medicare Part B claims data. Our conceptualization is based on, and consistent with, those of Steinwachs (1979), Saultz (2003), and Haggerty and colleagues (2003). Steinwachs identified four factors for consideration: time period (fixed or episodic), organizational context (medical specialty and site), provider (individual or team), and problem type (selected diagnosis or all care). Saultz (2003) identified a hierarchical definition progressing from informational continuity (when medical records are accessible by any health care team member), to longitudinal continuity (when a regular source [“medical home”] exists for each patient), and ultimately to interpersonal continuity (when the patient uses a particular physician for all basic health services). Haggerty and colleagues went to the heart of the issue by noting that continuity of care involves … the relationship between a single practitioner and a patient that extends beyond specific episodes of illness or disease … and implies a sense of affiliation between patients and their practitioners (my doctor or my patient), often expressed in terms of an implicit contract of loyalty by the patient and clinical responsibility by the provider. (p. 1219) Simply put, our conceptualization assumed that continuity of care involves an ongoing relationship with a particular physician in the outpatient setting with sufficient frequency for that physician to assume primary responsibility for both the patient’s basic health care needs and his or her overall disease and care management. That having been said, it is important to note here a salient limitation of our approach. The operational definition of continuity of care that we advance in this article does not require evidence that the physician actually assumed primary responsibility for his or her patient. Because we developed our measure to rely solely on the information contained in Medicare claims, evidence of the assumption of that primary responsibility was simply not available. Thus, although our measure is consistent with the heart of the continuity of care issue as described by Haggerty and colleagues (2003), it only requires that there were a sufficient frequency of visits to the same physician for this to have occurred.