While adverse outcomes linked to anesthesia are uncommon in healthy patients, they do occasionally happen. There is rarely a single cause. Anesthesia and surgery bring the patient into a complex world in which innumerable small failings can converge to produce an eventual catastrophe. And for all the technology involved, the anesthesiologist remains the cornerstone of safe anesthesia care, protecting the patient from harm regardless of its source. Responding to the demands of the operating room environment requires on-the-spot decision making in a complex, uncertain, and risky setting. Only responsible, professional human beings acting in concert can perform this task; no machine that we devise now or in the foreseeable future will suffice. I have outlined the components of a dynamic decision-making process that successfully protects patients in almost all cases. However, being human, anesthesiologists do make errors along the way--errors we are just beginning to understand. Sometimes these errors are due to faulty vigilance or incompetence, but usually they are made by appropriately trained, competent practitioners. Anesthesiologists can err in many ways, and recognizing these ways makes it easier to analyze the events leading to an anesthetic accident. More importantly, it better equips us to eliminate or minimize them in the future--and this is the real challenge.