Several of the vulnerability assessment techniques described in Chapter 4 related to geographically based mapping of calculated numerical indices (Committee on Techniques for Assessing Groundwater Vulnerability, 1993). The purpose of this chapter is to describe the formulation and testing of a specific Nitrate Pollution Index for groundwater in agricultural areas in the U.S. The chapter begins with conceptual information related to the development of an index. The major focus of the chapter is on the developed index and its testing via the use of groundwater nitrate data for three geographical areas in the U.S. The chapter concludes with a summary of the advantages and limitations of the developed Nitrate Pollution Index.