Research Findings: A 9-month study served to evaluate the effectiveness of a pre-kindergarten number sense curriculum. Phase 1 of the intervention involved manipulative-, game-based number sense instruction; Phase 2, computer-aided mental-arithmetic training with the simplest sums. Eighty 4- and 5-year-olds at risk for school failure were randomly assigned to (a) structured discovery of the n+0/0+n=n pattern and the n+1/1+n = the number after n relation; (b) structured discovery with explicit instruction; (c) blocked practice of (zero, one, and number-after) items; and (d) haphazard practice. Analyses with a Wilcoxon signed-rank test of follow-up Test of Early Mathematics Ability-Third Edition and mental-arithmetic testing indicated that general achievement and fluency with n+0/0+n combinations improved significantly. Significant improvement for n+1/1+n combinations was evident only if success included slow or counted answers. Practice or Policy: Theoretical, methodological, and educational implications are discussed, including the need to "score in context" (e.g., consider responses to other items). (Contains 13 tables and 2 figures.)