This study investigates the nature of human errors in the intensive care unit (ICU), adopting approaches developed by human factors engineering. The methodology, referred to as task analysis, was used to interpret the activities around patients in a medical-surgical ICU. A team of specially trained nonmedical investigators observed daily activities, while physicians and nurses simultaneously reported any observed errors. Based on the pooled data, an estimated 1.7 errors per patient-day occurred, with nearly 2 severe or detrimental errors occurring in the ICU as a whole. The methods explored in this study represent a growing trend in improving safety, which is to better understand the true nature of errors in complex health care settings such as an ICU. The authors conclude that communication failures between physicians and nurses play a significant role in the described errors, a problem they emphasize should be amenable to intervention.