The development of the Individualized Mathematics (IM) program is described in this paper. This program, for use in grades K-3, incorporates elements from predecessor Learning Research and Development Center mathematics programs, IPI mathematics, and PEP quantification. It features the systematic use of lessons involving manipulatives in an individualized program and the gradual introduction of paper-and-pencil lessons. The paper describes the overall structure of the program, including lesson and test materials and classroom management procedures. It provides a rationale for the mathematics taught in the program by listing the pertinent descriptions and definitions associated with selected mathematical concepts, together with examples of instructional objectives derived from such definitions. Selected evaluation data are provided, and some conclusions concerning the general tasks of development and implementation are discussed. (Author/DT)