Colleges and universities continue to experience growth in their online programs, yet retention, persistence, and graduation rates of adult online learners remain problematic. However, only some higher education institutions offer professional development specifically on teaching adult learners online to address the long-standing issue. This modified Delphi study aimed to determine consensus from expert panelists on the learning gaps in professional development for faculty teaching adult learners online and if professional development for faculty on adult learning could improve online instruction and course design. A faculty professional development curriculum outline, titled "Practical Approaches to Teaching Adult Learners: Integrating Adult Learning Theories," was designed to address the gap in the research. The mixed methods study contained quantitative and qualitative questions and consisted of 15 expert panelists serving at Council for Christian Colleges & Universities institutions across the United States. The study concluded after three rounds because consensus was reached, and the research questions were answered. The study results were divided into three categories, including learning gaps in professional development, the need for ongoing and prerequisite training on adult learning theories, and the evaluation of the professional development curriculum proposed in the study. Notably, consensus was reached that the faculty professional development curriculum addressed the learning gaps, can improve online learning and course design, is effective and beneficial, and is ready for broad implementation. Future research recommendations and implications for practice were also discussed to assist colleges and universities in increasing faculty support, student satisfaction, and the long-term success of higher education institutions. [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.]