Number sense is easy to recognize but difficult to define and hence, to teach. In this article, number sense is defined in terms of the knowledge known to underline it, as identified in cognitive developmental theory and research. A preK-2 mathematics program, called Number Worlds, that was specifically developed to teach this knowledge is described and five instructional principles that lie at the heart of the program, and that are grounded in recent theories of learning and cognitive development, are used to illustrate several of its features. Evidence of the program's effectiveness in teaching number sense and in enhancing the mathematics learning and achievement of at-risk children is briefly presented.