The purpose of this study was to determine the effectiveness of online professional development training on assistive technology by comparing knowledge acquisition through pretest/posttest scores and assessment of the participants' assistive technology implementation plans with a researcher designed rubric. The architecture for the six trainings was built using technology-based instructional modules including real-world, problem-based instruction with multimedia video and student examples. The six-week professional development course was offered by the College of Education at a public university in the intermountain western United States. This study used a mixed methods analysis including a one-group pretest-posttest design. Specifically, the five data collection procedures were: (1) a researcher-created achievement instrument; (2) the DELES survey instrument; (3) follow-up focus group questions; (4) a researcher-created AT plan rubric; and, (5) a survey data from expert panels. Results from the data analysis indicated there was a significant difference for the pretest and posttest conditions. The distribution of scores in the DELES, and as further communicated in the Focus Group Questions, indicated the in-service teacher participants had a positive attitude toward the online professional development course. The participants' rubric scores from their assistive technology (AT) plans (assessed posttreatment) fell within the advanced range indicating they exhibited skills in assessing and implementing assistive technology. The development of the online training met the ADDIE criteria as validated by the SMEs and IDEs through a modified Delphi technique. The major outcome from the study is the confirmation that the online professional development approach was an effective and much needed alternative approach to face-to-face implementation, particularly for a large geographic and rural state such as Idaho. Further, the method utilized in the training may lead to future online training for Idaho educators to fill a possible gap in teacher professional development within the State. [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.]