: Nurses are responsible for the safety of their patients. Recent discussions in healthcare have questioned the effect of work hours and fatigue on patient safety. Nursing research has found that over half of the reported errors and near errors involve administration of medications. The likelihood of making an error increases when nurses work extended ? durations, overtime, and many hours per week. Nurses also report that working longer shifts leads to greater fatigue and reduced work quality. With medication errors accounting for over 7,000 deaths per year, research in the field of medication safety is needed. This research contributes directly to the AHRQ mission because greater knowledge of medication errors and the medication administration process will improve patient safety, thus improving the healthcare patients receive. The objectives of this research are to examine the relationship between nurses' work hours and the occurrence of medication administration errors and to examine the relationship between fatigue in nurses and the occurrence of medication administration errors. This non-blinded, observation-based study is the first in-depth, observation-based examination of nurses' work hours and the occurrence of medication administration errors. It will be conducted at one Midwestern academic medical center and one ? Midwestern community hospital. The medication administration process will be observed by trained observers in nurses for three, 2-hour periods of time over the course of a scheduled shift. Observation will provide the medication administration error data for determining the error rate per nurse. Eligible registered nurses for the study will work in either the ICU or the ED, depending on the unit selected through pilot analysis. Nurses will consent to participate in this study. In addition to the observation data, this study will collect data through three questionnaires that each nurse will complete. These ? questionnaires include personal items, work-related items, a chronic fatigue scale, and an acute fatigue scale. ? ? ?