Improving the daily practice of medicine requires making changes in processes of care.In many circumstances, the most powerful way to make such changes is to conduct small, local tests-Plan-Do-Study-Act (PSA) cycles-in which one learns from taking action. Learning in these cycles has much in common with learning from prudent clinical work, in which therapies are initiated under close observation and adjustments are made as data and experience accumulate. For many system improvements, PSA cycles are more appropriate and informative than either formal studies with experimental designs (such as randomized trials) or the mere implementation of changes without reflection or evaluative measurement. Physicians can encourage systemic improvement by endorsing and participating in prudent, local tests of change in their own offices and in the health care organizations in which they work. To do this, they must understand the scientific value and integrity of such small-scale tests.