In this chapter, the basic concepts of Human Factors (HF) as well as their relationship with patient safety (PS) in anesthesiology are presented. A model of cognitive functioning is described in order to understand how individuals handle critical situations and routine tasks, and, moreover, “human error” is defined. Concepts are exemplified with concrete anesthesiology cases. A model of cognitive functioning will be described so as to comprehend how individuals operate efficiently when facing different situations and tasks. Then, this theoretical framework will be used to define and categorize what is known as human error, by trying to exemplify with concrete cases of tasks that are related to anesthesiology, with a view to reinforcing the understanding of each concept addressed. To continue, two models for the analysis of safety events are evaluated: the linear model and the systemic model. The former’s limitations are highlighted and the epidemiological/systemic analysis model is introduced as an evolutionary, more comprehensive and complementary alternative. Emphasis is placed on the fact that “human error” is considered a “cause” in the linear model, whereas the second model regards it as a “symptom” and seeks to identify its causes. This conceptualization of human error will be essential when it comes to understanding, designing and implementing effective actions – barriers – that allow a significant reduction in the likelihood of recurrence of similar events. Thus, highlighting the crucial role played by defensive barriers in robust systems across different anesthesiology work situations from the point of view of patient safety.