The chapter opens emphasizing the importance of numeracy: Numeracy is the new literacy. The chapter introduces the fundamental concepts of numeracy and the incidence and the nature of dyscalculia. We examine the important issues: (a) What constitutes numeracy and its importance not only to school mathematics, but also to higher mathematics? (b) Why individuals experience difficulty in mastering numeracy: Number Concept, number relationships (arithmetic facts), and outcome of numerical operations. (c) Why do many children have difficulty mastering numeracy? (d) What role does instruction play in children’s difficulties in numeracy? and (e) How do deficits in numeracy skills affect individuals’ lives in multiple ways? In the second part of this chapter, we discuss the multiple perspectives that guide our understanding of learning problems in mathematics, particularly dyscalculia and acquired dyscalculia.