Although the early development of children's number sense is a strong predictor of their later mathematics achievements, it has been overlooked in primary schools in Malaysia. Mainly attributable to underdeveloped number sense of Malaysian primary and secondary school children, their inability to handle simple mathematics tasks, which require the understanding of basic mathematical concepts, numerical estimation, and mental computation, is a cause for worry. To enhance the perception of curriculum designers and mathematics teachers about why and how number sense ought to be developed, this article serves as a review of essential components, theoretical framework, and test of number sense.