The purpose of this study was the development and validation of an instrument to measure the self-efficacy of elementary mathematics teachers. Self-efficacy, as defined by Bandura, was the theoretical framework for the development of the instrument. The complex belief systems of mathematics teachers, as touted by Ernest (1989) provided insight into the elements of mathematics beliefs that could be relative to a teacher's self-efficacy beliefs. The Self-efficacy for Teaching Mathematics Instrument (SETMI) was developed in August 2010 and has undergone revisions to the original version through processes defined in this study. Evidence of reliability and validity were collected to determine if the SETMI is an adequate instrument to measure self-efficacy of elementary mathematics teachers. Findings indicate that reliability of the instrument is adequate but that the original constructs represented by the instrument may be different than initially anticipated. Construct validity of the revised SETMI was tested using Confirmatory Factor Analysis and while the measurement models fit the covariance matrix, the items that represent mathematics content may in fact be better measures of self-concept for mathematics content knowledge. These findings indicate evidence of a potential structural model for self-efficacy.