Number relationships, which go far beyond counting skills, refer to the ability to represent a quantity in multiple, flexible ways. It is arguably among the most important mathematics concepts in number and quantity. The current study examined the effectiveness of number relationships instruction in preschool classrooms. Participants included 73 children and 4 teachers from a half-day preschool program in a local school district. For the intervention group, two teachers provided number relationships instruction to 37 of the children in their classrooms (four sections total). No treatment occurred for the control group consisting of the remaining 36 children taught by two teachers. Before and after the 12-week treatment period, the TEMA-3 (Test of Early Mathematics Ability-3rd Edition) was administered both as a pretest and a posttest to assess children's understanding of number and quantity. Results indicated that children in the intervention group who received mathematics instruction with the emphasis on teaching number relationships scored significantly higher on the posttest than their counterparts in the control group. However, results of the current study did not reveal any advantages by age group for number relationships instruction. Small sample size may have limited this analysis. (Contains 5 tables and 1 figure.)