OBJECTIVES: To describe the types and potential clinical significance of medication administration errors in assisted living (AL). DESIGN: Cross-sectional observational study. SETTING: This study was conducted in 12 AL settings in three states (Oregon, Washington, and New Jersey). PARTICIPANTS: Participants included 29 unlicensed assistive personnel and 510 AL residents. MEASUREMENTS: Medication administration observations, chart review, and determination of rates, types, and potential clinical significance of errors using standardized methodology. RESULTS: Of 4,866 observations, 1,373 errors were observed (28.2% error rate). Of these, 70.8% were wrong time, 12.9% wrong dose, 11.1% omitted dose, 3.5% extra dose, 1.5% unauthorized drug, and 0.2% wrong drug. Excluding wrong time, the overall error rate dropped to 8.2%. Of the 1,373 errors, three were rated as having potential clinical significance. CONCLUSION: A high number of daily medications are given in AL. Wrong time accounted for the majority of the errors. The bulk of the medications are low risk and routine; to promote optimal care delivery, clinicians need to focus on high-risk medications and residents with complex health problems.