A child's demonstration of his conceptual understanding of number bodes well for his future success in school mathematics. As youngsters' thinking becomes more logical, they apply one-to-one correspondence relationships to quantification. Yet, reliable assessment of young children's mathematical ability is difficult because of social and emotional factors related to their development. The Arlitt Preschool Curriculum-Based Measure for Math (APCBM-Math) can be used for required monitoring of children with diagnosed disabilities as well as for program accountability. It is an interactive game that is played between assessor and child. The game consists of a basket of teddy bear counters and a deck of cards, prearranged in a standardized order for consistency. The use of concrete objects is an important component of the assessment because it allows the assessor to document that children understand underlying number sense concepts, such as set representation and comparison (NCTM 2000). In this article, key number sense concepts that teachers should look for as they interact with young children are mentioned. (Contains 1 table and 2 figures.)