Reasoning ability refers to the power and effectiveness of the processes and strategies used in drawing inferences, reaching conclusions, arriving at solutions, and making decisions based on available evidence. The topic of reasoning abilities is multidisciplinary—it is studied in psychology (differential and cognitive), education, neuroscience, genetics, philosophy, and artificial intelligence. There are several distinct forms of reasoning, implicating different reasoning abilities. Deductive reasoning involves drawing conclusions from a set of given premises in the form of categorical syllogisms (e.g., all x are y) or symbolic logic (e.g., if p then q). Inductive reasoning involves the use of examples to suggest a rule that can be applied to new instances, invoked, for example, when drawing inferences about a rule that explains a series (of numbers, letters, events, etc.). Abductive reasoning involves arriving at the most likely explanation for a set of facts, such as a medical diagnosis to explain a set of symptoms, or a scientific theory to explain a set of empirical findings. Bayesian reasoning involves computing probabilities on conclusions based on prior information. Analogical reasoning involves coming to an understanding of a new entity through how it relates to an already familiar one. The related idea of case-based reasoning involves solving a problem (a new case) by recalling similar problems encountered in the past (past cases or stored cases) and using what worked for those similar problems to help solve the current one. Some of the key findings on reasoning abilities are that (a) they are important in school, the workplace, and life, (b) there is not a single reasoning ability but multiple reasoning abilities, (c) the ability to reason is affected by the content and context of reasoning, (d) it is difficult to accelerate the development of reasoning ability, and (e) reasoning ability is limited by working memory capacity, and sometimes by heuristics and strategies that are often useful but that can occasionally lead to distorted reasoning. Several topics related to reasoning abilities appear under different headings, such as problem solving, judgment and decision-making, and critical thinking. Increased attention is being paid to reasoning about emotions and reasoning speed. Reasoning ability is and will remain an important topic in education.