This chapter explores the cognitive processes of reasoning and problem-solving, which are central to decision-making and the navigation of complex situations. It differentiates between deductive reasoning—applying general principles to specific cases—and inductive reasoning—drawing general conclusions from specific instances. The chapter delves into heuristics, the mental shortcuts that facilitate quick judgments, and the cognitive biases, such as availability, representativeness, and anchoring, that can lead to systematic errors. Real-world examples illustrate how these processes influence everyday decisions and even extend to artificial intelligence, where inherited biases from training data can have significant repercussions. Additionally, the chapter outlines various problem-solving strategies, from trial and error to working backward and analogical reasoning, and highlights the role of insight and creativity in generating innovative solutions. It concludes with a discussion on expertise, emphasizing how sustained practice refines cognitive skills, enabling faster and more accurate problem resolution in specialized domains.