Adherence to therapy is defined as the extent to which a person's behavior in taking medication, following a diet, and/or executing lifestyle changes, corresponds with agreed recommendations from a healthcare provider. Patients presenting with type 2 diabetes mellitus are initially encouraged to maintain a healthy diet and exercise regimen, followed by early medication that generally includes one or more oral hypoglycemic agents and later may include an injectable treatment. To prevent the complications associated with type 2 diabetes, therapy frequently also includes medications for control of blood pressure, dyslipidemia and other disorders, since patients often have more than three or four chronic conditions. Despite the benefits of therapy, studies have indicated that recommended glycemic goals are achieved by less than 50% of patients, which may be associated with decreased adherence to therapies. As a result, hyperglycemia and long-term complications increase morbidity and premature mortality, and lead to increased costs to health services. Reasons for nonadherence are multifactorial and difficult to identify. They include age, information, perception and duration of disease, complexity of dosing regimen, polytherapy, psychological factors, safety, tolerability and cost. Various measures to increase patient satisfaction and increase adherence in type 2 diabetes have been investigated. These include reducing the complexity of therapy by fixed-dose combination pills and less frequent dosing regimens, using medications that are associated with fewer adverse events (hypoglycemia or weight gain), educational initiatives with improved patient-healthcare provider communication, reminder systems and social support to help reduce costs. In the current narrative review, factors that influence adherence to different therapies for type 2 diabetes are discussed, along with outcomes of poor adherence, the economic impact of nonadherence, and strategies aimed at improving adherence.