The consumer price index is probably the most widely used measure of inflation. Changes i n the index affect the incomes of a substantial portion of the U.S. population. This report provides background information on the history and concepts of the index. In addition, several factors which may produce biases in the index are analyzed. The objective is to provide an introduction to the CPI f o r the policy maker who wishes to acquire a working knowledge of the concept as an aid in examining economic policy alternatives.