The purpose of this mixed-methods research study was to examine how stress levels of middle school mathematics teachers who taught Algebra I in school districts in the state of Pennsylvania relate to high-stakes testing and to explore the experiences of middle school mathematics Algebra I teachers. The researcher collected and compared it to normed data from a quantitative survey instrument called the Educator Test Stress Inventory (ETSI). The participants consisted of Algebra I teachers from public middle schools in the state of Pennsylvania. Data was collected at two intervals, once in August and once in December. Quantitative survey data was downloaded into SPSS statistical package for analysis. Independent t tests, a paired sample t test, and Pearson's Coefficient were calculated to analyze for correlations between designated variables. There was a significant difference in manifestation stress value scores between gender on the first administration of the ETSI as well as the normed data of the ETSI. There was not a significant difference between administrations of the ETSI, nor was there a correlation of stress values to teaching years of experience. Ten semi-structured interviews of middle school Algebra I teachers were conducted to explore their experiences in a phenomenological approach. The constant comparative method was employed for the purpose of establishing emergent codes to develop themes and patterns. Four major themes emerged from the qualitative data: love for middle school, curriculum concerns, stress, and high-stakes testing and teacher stress. This study addressed the gaps in the literature on the topic of high-stakes testing and its impact on the stress levels of middle school mathematics teachers. Additionally, this study added to the field of professional knowledge on this topic by investigating high-stakes testing and stress levels of middle school mathematics teachers via the administration of the ETSI. [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/en-US/products/dissertations/individuals.shtml.]