High blood pressure continues to be a highly prevalent condition affecting about 20 percent of the U.S. population. Untreated, it may lead to coronary heart disease, stroke, kidney failure, and other complications. Medication is a major means of controlling hypertension and averting these debilitating consequences. Treatment with drugs, however, is undermined by suboptimal adherence to therapy. Forgetting to take medications is the most commonly elicited reason for lapses in medication use; but side effects, health beliefs, lack of knowledge of disease and treatment instructions, and financial barriers may also interfere. Determining those factors that (1) are predictably associated with fluctuations in adherence (such as day of the week, holidays, medication appointments); (2) ameliorate the impact of nonadherence, such as pharmacologic properties of drugs that protect patients during adherence lapses; and (3) correlate with the accuracy of adherence reporting by patients will assist in anticipating and addressing adherence obstacles and serve as the objectives of the study.