Adverse effects related to health care are common and partly avoidable. We need to identify how and why adverse events occur and how system defects may contribute to their occurrence. Systems and processes can be designed to help prevent errors and decrease harm that occurs when they are not intercepted. Tactics to reduce errors and mitigate their adverse effects include reducing complexity and optimizing information processing. Implementation of information technology may offer great promise but the most important is to make an effort to promote a culture of safety.