The purpose of this quantitative correlational-predictive study was to determine the extent to which participation in science, technology, engineering, and mathematics professional development (STEM PD) predicted technology proficiency in K-12 teachers who completed 8-40 hours of STEM PD in the southwestern United States, while controlling for teaching experience, education level, and technology industry experience. The theories of social constructivism, instructional practices in integrated STEM, community of inquiry, and technological pedagogical content knowledge provided the theoretical framework for this quantitative study. The convenience sample consisted of 216 K-12 teachers in the southwestern United States. The 34-item Technology Proficiency Self-Assessment for 21st Century Skills instrument measured technology proficiency in K-12 teachers. Participation in STEM PD was measured as number of hours. The results of a hierarchical linear regression showed that the complete model, including three controls and STEM PD, significantly predicted technology proficiency, adj. R[superscript 2] = 0.188, F (4, 211) = 13.463, p < 0.001. Participation in STEM PD significantly increased the variance accounted for in the complete model, R[superscript 2][subscript change] = 0.095, F[subscript change] (1, 211) = 25.218, p < 0.001. These results supported the effectiveness of participating in STEM PD in terms of K-12 teachers' technology proficiency. The study findings provide an additional argument for schools and school districts to encourage and support K-12 teachers' participation in STEM PD, which, among other benefits, contributes to teachers' technology proficiency. Keywords: STEM PD, technology proficiency. [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.]