Hepatitis C virus (HCV) is the most common chronic blood-borne infectious disease in the United States. The Centers for Disease Control and Prevention estimated that 16,000 Americans were newly infected in 2009, and between 2.7 and 3.9 million community-dwelling people were living with chronic HCV infection. The primary goal of chronic HCV detection and treatment is to prevent complications and death from HCV infection. Response to HCV treatment is typically defined by surrogate virological measures, such as sustained viral response (SVR) and early viral response (EVR). Studies have shown that a variety of factors affect treatment response, including viral or disease-related factors; treatment related factors, such as the dose and duration of treatment and treatment history; and patient related factors, such as age, race/ethnicity, comorbidities, and presence of fibrosis. Genotyping is among the best ways to predict viral response to treatment and is used to determine treatment type and duration. Randomized evidence has demonstrated that antiviral therapies are efficacious in the treatment of chronic HCV infection. When it comes to effectiveness and quality of care, however, a number of issues, including treatment adherence, need to be addressed. Adherence to HCV treatment is challenging because of the lengthy duration, complex treatment regimen, and frequent adverse events. Adherence challenges are likely to become even more significant with the introduction of triple therapy. Several observational studies have examined the association between adherence and treatment outcomes, particularly SVR, in hepatitis C patients. The existing body of literature consistently shows that increasing adherence to dual therapy is associated with improved likelihood of achieving SVR. Therefore, efforts are needed to improve treatment adherence in HCV. Adherence, in the context of HCV treatment, includes patient adherence to both the medication regimen and the overall medical plan. Medication adherence is defined as the patient's use of antiviral agents according to the prescribed dose, duration, frequency, and timing. In contrast, medical plan adherence indicates that patients complete followup visits, laboratory tests, or other medical procedures according to the physician's directions. In this report, we refer to adherence to medication and adherence to the overall medical plan during HCV treatment as patient adherence, or "adherence" more generally. Nonadherence to HCV treatment may be associated with a lack of management of adverse events, higher pill burden and lengthy treatment, limited provider experience, active substance use, lack of social support, and presence of cirrhosis. Interventions for improving adherence can be categorized according to the primary risk factor they target: (1) policy-level interventions, (2) system-level interventions, (3) provider-level interventions, (4) regimen- or therapy-related interventions, (5) patient-level interventions, or (6) interventions designed to help manage adverse events. The Key Questions for this review are as follows. Key Question 1. In adult patients with chronic HCV infection undergoing antiviral therapy, what is the comparative effectiveness of treatment adherence interventions in improving intermediate (e.g., sustained viral response, histological changes, drug resistance, relapse rates, and treatment side effects) and health outcomes (e.g., disease-specific morbidity, mortality, QOL, transmission of HCV)? Key Question 2. What is the comparative effectiveness of treatment adherence interventions in improving treatment adherence (e.g., medication adherence, medical plan adherence)? Key Question 3. What are the harms associated with hepatitis C antiviral treatment adherence interventions?