The job of an analyst is to make sense of a complicated mass of information—to understand and explain the current situation, to reconstruct the past that led to it, and to use it as the basis of predictions for the future. To do so requires many types of sophisticated reasoning skills. This chapter first describes a prominent historical characterization of overall individual human reasoning—that reasoning is filled with “irrationalities.” The chapter then remarks on more recent characterizations of reasoning that try to uncover the judgment mechanisms that produce these irrationalities, including recognizing that human reasoning might best be thought of as involving both unconscious and conscious components that have different strengths and weaknesses. Finally, it describes two important characteristics of reasoning abilities: that people seek coherence, and that people are particularists (i.e., that we tend to emphasize the uniqueness of each situation). The chapter illustrates how these characteristics apply in several general tasks involved in analysis, including interpreting questions, searching for information, assessing information, and assessing our own judgments.