Despite improvements in the control of hypertension in the United States, there is evidence that considerable numbers of people still have uncontrolled high blood pressure (BP). Because mortality and morbidity are reduced in proportion to BP reduction, it is important to identify the major causes of failure to control hypertension. This article summarizes the results of various studies of BP control: population surveys, assessments of usual practice, worksite programs, large-scale special projects, and quality assurance programs. The evidence suggests that patient characteristics and behaviors explain much of the poor BP control in patients under care. The most important of these patient factors is compliance. However, other factors associated with poor BP control are excessive alcohol intake, life stress, lower income, younger age, black race, male sex, and unmarried state. In evaluating BP control, the clinician and researcher must also be aware of various measurement effects that may lead to erroneously high BP readings.