The problem addressed through this study was the inconsistent implementation of online formative assessment to inform instruction by high school mathematics teachers in a Northeast Regional High School District (NRHSD). The purpose of this basic qualitative study was to investigate how high school mathematics teachers in a NRHSD use online formative assessment to inform instruction. Identification of successful online formative assessment can be used to inform the training, resources, and supports teachers need to promote student achievement. The technology acceptance model (TAM) was used as the conceptual framework to ground the project study. Interviews with 12 high school mathematics teachers were conducted to gather descriptive data on how teachers use online formative assessment to inform instruction. The data from the interviews were used to answer two research questions: "What are teachers' perspectives on ease of use, usefulness, and attitudes towards online formative assessment to inform high school mathematics instruction", and "What are high school mathematics teachers' recommendations regarding the training, resources, and support they need to consistently use formative assessment to inform their instruction." The results of the study indicated that using online formative assessment data to inform instruction could promote positive social change in high school mathematics classes by increasing student engagement and academic achievement. Data collected from interviews indicated that teachers need to focus on one program with professional development targeted to user level. Furthermore, putting mathematics teachers' perceptions and use of technology data into action could lead to graduating students with the skills needed to positively contribute to society. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]