This study explored (i) whether there might have been changes in teachers' iSTEM self-efficacy after the completion of a Massive Open Online Course (MOOC) on integrated Science, Technology, Engineering, and Mathematics (iSTEM) education models, (ii) whether any changes in iSTEM self-efficacy were related to teachers' years of experience, sex, or having previous experience teaching iSTEM, and (iii) whether a correlation existed between teachers' iSTEM self-efficacy scores and the number of main characteristics of the iSTEM educational model they incorporated in the unit plan they created as part of the course. Based on Mobley's (2015) SETIS instrument for science teachers, a survey was adapted and administered via Google Forms to measure teachers' iSTEM self-efficacy before and after completing the MOOC. Pre- and post-course within-subjects differences were studied through a mixed design repeated measures ANOVA, and a correlational analysis was used to look at the relationship between iSTEM self-efficacy and the level of alignment of participant-generated unit plans with the main features of the iSTEM model. Results showed a general increase in iSTEM self-efficacy, an interaction with teaching experience, and also a positive correlation between iSTEM self-efficacy and unit plans' level of alignment with iSTEM characteristics. This supported the improvement of the online course on STEM integration directly, and provided knowledge on iSTEM self-efficacy with potential practical applications for educators, such as the possibility of increasing teachers' level of confidence for integrating STEM subjects through the completion of self-paced MOOCs, and the correlation between iSTEM self-efficacy and the level of integration shown in teacher-generated unit plans. [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.]