University instructors often lack the knowledge and skills to effectively facilitate learning and teaching in the online environment. Instructors in higher education institutions (HEIs) have faced common challenges to online teaching, such as a lack of digital literacy, access to software and hardware resources and training, and pedagogical and content-related concerns. The rapid transition to fully online teaching necessitated by the onset of the COVID-19 pandemic brought many of these instructional issues to the foreground; however, online teaching also encompasses the integration of online tools into traditional classroom environments, as well as blended or hybrid instructional formats. In this dissertation, I define online teaching broadly to include all such formats of online teaching. In this multiple-case study, I aimed to investigate the online teaching challenges of higher education instructors in Hong Kong and to examine the approaches to professional learning these instructors took in order to address the challenges they encountered. I employed qualitative methods in this study, including conducting individual semi-structured interviews with instructors working at a tertiary university in Hong Kong that specializes in teacher education and collecting artifacts from the participants that related to their professional learning experiences. I found that instructors experienced online teaching challenges that encompassed content-related, pedagogy-related, technology-related, and motivational factors, thereby providing insights for both practice and future research into improving online teaching competencies and experiences for instructors. My findings highlight the importance of integrating both formal and informal professional learning approaches to address online instruction challenges. Formal professional development (PD) programs, such as university workshops and certificate courses, provided structured learning opportunities for instructors. Informal PD through social media platforms and professional networks facilitated collaborative learning and knowledge sharing, and was often engaged in to supplement formal PD. The integration of both formal and informal PD approaches was suggested to provide a comprehensive and personalized professional learning experience. Furthermore, I adopted Activity Systems Analysis (ASA) as a framework to analyze my findings regarding the professional learning strategies of higher education instructors in response to their online teaching challenges. The research implications of ASA included investigating the compensatory role of informal PD, understanding the time-intensive nature of informal PD tasks, exploring customization possibilities for software tools, and examining the impact of mandatory informal PD on instructors' workloads. [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.]