This section introduces the concepts covered in the chapter about Mathematical Reasoning using illustrations, the process of reasoning, different kinds of reasoning, fundamentals of deductive reasoning. This chapter discusses some basic ideas of Mathematical Reasoning, especially in the context of Mathematics. Students, by now, must have already learnt inductive reasoning in the context of Mathematical Induction. In this chapter, some fundamentals of deductive reasoning are covered. The topics present in the chapter include mathematically acceptable statements, connecting words/ phrases – consolidating the understanding of “if and only if (necessary and sufficient) condition”, “implies”, “and/or”, “implied by”, “and”, “or”, “there exists”, validating the statements involving the connecting words, and more.