This study investigated the development of the concept of variables in middle grades mathematics textbooks during four eras of mathematics education in the United States. Findings revealed that each of the middle grades mathematics curricula examined used variables, but in varied proportions and levels of complexity. Formal definitions for variables were found in 11 of the 12 textbooks examined. The characteristics of the definitions for variables found in the different curricula were, however, different from one another. The uses of variables as a "specific unknown quantity" and as a "label" dominated the uses of variables in the mathematics curricula. The least used category of variables was as an "abstract symbol." Overall, the data did not reveal any systematic or drastic change in the treatment of variable ideas during the 50 year period within which the study was situated. There was however, a steady increase in the use of variables as "varying quantities" across grade levels, and across the four eras of mathematics education in the United States. There were also some noticeable changes in the treatment of variable ideas found in the curriculum selected for the present NCTM era when compared with the treatment in the other three curricula. (Contains 5 tables and 6 figures.)