Hundreds of thousands of K-12 children in the United States are enrolled in online K-12 virtual schools that consistently report poor academic outcomes. There is a need to assess how well instructors in a synchronous online environment present new material to learners in a way that best aligns with how the brain manages and integrates new information into long-term memory. Online K-12 teachers use PowerPoint to design Electronic Slide Presentation (ESP) decks, which are used as their main form of instruction with their students during synchronous classes. The Cognitive Theory of Multimedia Learning (CTML) provides a set of principles which are proven to reduce extraneous cognitive processing, manage essential processing, and foster generative processing for learners. Yet many are concerned that teachers lack the skills and knowledge of best slide deck design practices required to create effective online learning environments. This research examines online K-12 teachers' perceptions and practices related to designing ESP slides that mitigate extraneous cognitive load. This study establishes a base of knowledge previously unknown about online teacher practices to determine if there is a need for teacher education or professional development materials specific to improving synchronous K-12 virtual classroom learning outcomes in the context of ESP design. The purpose of the study was to investigate to what extent virtual K-12 teachers design their lesson slides to reduce cognitive overload for their students. A questionnaire was used to measure perceptions and practices of teachers at a large K-12 academy encompassing three schools in the Midwest state of Ohio. A rubric was then used to evaluate sample ESP decks submitted by teachers to assess adherence to the CTML principles known to reduce extraneous cognitive load. Collected demographic information was analyzed with frequencies, means, and standard deviations. Group differences were examined using t-tests and Analyses of Variance (ANOVA) tests. Associations among variables were examined with correlation and multiple linear regression tests. Results of this research might be used to support teacher education and development programs. [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.]