This unit is 1 of 12 developed for the university classroom portion of the Mathematics-Methods Program (MMP), created by the Indiana University Mathematics Education Development Center (MEDC) as an innovative program for the mathematics training of prospective elementary school teachers (PSTs). Each unit is written in an activity format that involves the PST in doing mathematics with an eye toward application of that mathematics in the elementary school. This document is one of four units that are devoted to the basic number work in the elementary school. In addition to an introduction to the unit, the text has sections on developing initial concepts in addition and subtraction, basic mathematical content in addition and subtraction, developing the basic addition and subtraction facts, algorithms and problem solving, and appendices that present an overview of cuisenaire rods and the properties of number systems. (MP)