The focus of this chapter is on the nature of mathematical and statistical reasoning. The chapter begins with a description of the general nature of human reasoning. This is followed by a description of mathematical reasoning as described by mathematicians along with recommendations by mathematics educators regarding educational experiences to improve mathematical reasoning. The literature on statistical reasoning is reviewed and findings from the general literature on reasoning are used to identify areas of statistical reasoning that students find most challenging. Statistical reasoning and mathematical reasoning are compared and contrasted, and implications for instruction and research are suggested.