Reasoning and mathematics skills are typically studied in separate literatures. Yet, these abilities are related at multiple levels. For example, understanding new concepts, selecting an appropriate strategy to solve a mathematics problem, or learning from feedback involves reasoning. There is also emerging evidence that the basic representations that underlie certain forms of reasoning and mathematics skills (e.g., ordering ability, creating a “mental line” representation, making comparisons) are overlapping. The main focus of this chapter is on how reasoning skills are affected by dyscalculia/mathematics difficulties, and how reasoning skills relate to mathematics performance. To address these questions, I first consider the cognitive profiles of individuals with dyscalculia. Dyscalculia is associated with a range of cognitive deficits (in working memory, long-term memory for order, spatial attention, etc.) that are not specific to the domain of numbers. It is essential to understand how cognitive abilities and resources might be affected in dyscalculia, as reasoning skills rely heavily on cognitive resources. I also give an overview of the various ways in which reasoning and mathematics abilities are related, and also consider the evidence regarding the extent to which reasoning abilities in individuals with mathematics difficulties are affected. These questions are important both from a theoretical and practical point of view, and one aim of the chapter is to highlight and discuss these implications. I also outline some important future directions for this line of work.