Poor compliance with rationally prescribed drug regimens attenuates benefits of treatment, making compliance a key link between process and outcome in ambulatory care. Compliance is defined as ‘the extent of correspondence between the patient's actual dosing history and the prescribed regimen’. Electronic monitoring methods reveal that >30% of patients omit many prescribed doses, irrespective of disease, prognosis, or symptoms. Some drugs are better able than others to maintain therapeutic action during the more common lapses in dosing. These are called ‘forgiving’ drugs; their duration of action is more than twice the prescribed interval between doses, allowing action to continue when one or more doses are missed. Forgiveness has limits, so long lapses in dosing will nullify action of any drug, with economic consequences that depend on the clinical consequences of lapsed action, or, with some drugs, rebound effects. These practical points have only come to light with the use of electronic monitoring of compliance, which avoids the biases created by tablet counts and other methods that make it easy for patients to censor evidence for omitted doses. All else being equal, the most forgiving drug in its class will be associated with the best outcomes, for it will be least impacted by prevalent poor and partial compliance.