This exploratory study sought to examine the relationship between teachers' use of formative assessment and their self-efficacy beliefs. Specifically, this study involved a quantitative analysis of the relationship between teachers' beliefs, knowledge base, and the use of formative assessment to make informed instructional changes and their perceptions of self-efficacy. A three-part online survey, which included demographic, assessment, and self-efficacy questions, was administered to examine how third, fourth, and fifth grade teachers (n = 79) in one southwest Florida public school district rate their assessment knowledge base and practices and how their knowledge and beliefs regarding formative assessment relate to their sense of self-efficacy. Convenience sampling was employed, and data were analyzed using descriptive and inferential statistics. The findings indicate that teachers frequently use formative assessments to make informed changes about classroom instruction and that they perceive these changes to be effective in raising mathematics achievement. Further, the findings show that the respondents are comfortable with their level of assessment knowledge and, overall, have a high sense of teacher efficacy. Finally, teachers' use of formative assessment to inform mathematics instruction in third, fourth, and fifth grade is positively correlated with their self-efficacy in relationship to assessment type, assessment knowledge, and effectiveness of assessments. Future studies should explore the relationships between teacher beliefs and an observation of teachers' use of formative assessment, in their decision-making processes and in enacted changes to classroom practice. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]