This Mixed Method Case Research Study (MMCSR) investigated the application of andragogical principles by adult fitness instructors at the Greater Louisville YMCA's. Utilizing the Modified Instructional Perspectives Inventory (MIPI) and a focus group, the study explored the instructional views of fitness instructors from an andragogical perspective. Specifically, the study aimed to: (a) determine the andragogical principles applied by instructors, (b) assess the impact of demographic factors, such as sex, years of experience, and certifications, on the application of these principles, and (c) document instructors' good teaching practices. The present study revealed no significant differences in andragogical principles application based on sex, years of experience, or certification status among the instructors. Overall, instructors placed significant value on trust, empathy, and member-centered learning methods, emphasizing the importance of accommodating the individual learning needs of adult learners. These findings underscore the pivotal role of personalized and empathetic teaching strategies in facilitating adult education in the fitness context. The study's implications for practice suggest that instructors should continue to cultivate trust and employ experience-based learning techniques, disregarding gender, and certification status as determinants of instructional quality. Future research directions are proposed to delve into mechanisms of fostering trust and sensitivity and exploring student-centered approaches over a more extended period to enhance adult fitness education. The study's limitations, including its small sample size and cross-sectional design, highlight the need for broader, longitudinal research to validate and expand upon these findings and recommendations. [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.]