As the patient safety field matures, there is increasing recognition of the need to incorporate human factors engineering methods into analyzing errors and developing solutions. These methods were used to investigate the types and frequency of medication errors in two intensive care units. Although existing medication safety interventions have mainly targeted errors at individual stages of the medication management process (e.g., computerized provider order entry [CPOE] to prevent prescribing errors), this study found that in many cases, errors occurred in an interdependent fashion at multiple stages of the process. For example, incorrect transcription of an order could then lead to a medication administration error. While CPOE is likely a solution for a significant proportion of errors, this study's results indicate a need for closed-loop systems that can minimize the risk of all types of medication errors.