Chronic diseases are the leading cause of illness, disability, and death in the U.S. Providing medical care for chronic illness is often complex, as patients require multiple resources, treatments, and providers. One strategy for improving care for chronic conditions is to develop programs that improve care coordination and implement care plans. Case management (CM) is one such supplemental service, in which a person takes responsibility for coordinating and implementing a patient's care plan, either alone or in conjunction with a team of health professionals. CM tends to be more intensive in time and resources than other chronic illness management interventions, and it is important to evaluate its specific value. CM is often utilized when the coordination and integration of care is difficult for patients to accomplish on their own. CM usually involves high-intensity engagement with patients, and case managers often adopt a supervisory role in comprehensively attending to patients' complex needs. Conceptually, a case manager can be seen as an agent of the patient, taking a "whole-person" (rather than solely clinical or disease-focused) approach to care, and serving as a bridge between the patient, the practice team, the health system, and community resources. The coordinating functions performed by a case manager include helping patients navigate health care systems, connecting them with community resources, orchestrating multiple facets of health care delivery, and assisting with administrative and logistical tasks. Case managers also can perform clinical functions, including disease-oriented assessment and monitoring, medication adjustment, health education, and self-care instructions. Such clinical functions are often the defining aspects of other chronic illness management interventions. In the context of chronic illness care, they are central to the role of a case manager, but a case manager also performs coordinating functions. The Agency for Healthcare Research and Quality (AHRQ) commissioned this review to examine the evidence for the effectiveness of CM programs for chronic illness patients with complex care needs. Specifically, we considered interventions in which case managers had a substantive role in performing both clinical and coordinating functions. This report summarizes the existing evidence addressing the following Key Questions: KQ1: In adults with chronic medical illness and complex care needs, is case management effective in improving: a. Patient-centered outcomes, including mortality, quality of life, disease-specific health outcomes, avoidance of nursing home placement, and patient satisfaction with care? b. Quality of care, as indicated by disease-specific process measures, receipt of recommended health care services, adherence to therapy, missed appointments, patient self-management, and changes in health behavior? c. Resource utilization, including overall financial cost, hospitalization rates, days in the hospital, emergency department use, and number of clinic visits (including primary care and other provider visits)? KQ2: Does the effectiveness of case management differ according to patient characteristics, including but not limited to: particular medical conditions, number or type of comorbidities, patient age and socioeconomic status, social support, and/or level of formally assessed health risk? KQ3: Does the effectiveness of case management differ according to intervention characteristics, including but not limited to: practice or health care system setting; case manager experience, training, or skills; case management intensity, duration, and integration with other care providers; and the specific functions performed by case managers?