Despite the importance of professional development (PD) and recent government efforts, PD provision for Vietnamese teachers living outside major cities remains a challenge due to distance and costs. Mobile learning offers a potential solution thanks to its capacity to enable learning anytime, anywhere, and this study investigated its use to provide pronunciation PD for Vietnamese tertiary TESOL teachers, since the development of oral skills is considered problematic in this context. This study aimed to identify the essential conditions and principles for mobile learning as an effective PD provision method in the Vietnamese context. To do so, it investigated: 1. factors influencing participation and learning experience; 2. attitudes, behaviours and engagement patterns regarding self-directed mobile assisted language learning; and 3. strategies to facilitate learning for different groups of learners. Design-based research was employed as the research paradigm and data collection involved iterations of an online PD pronunciation course over a two-year period through a questionnaire, pre and post test, interview and records of app usage, with the interview being the primary and most important source for data analysis. The participants were 57 TESOL teachers from 46 Vietnamese higher education institutions. Data analysis revealed five groups of factors influencing participation and learning experience relating to the learner, personal context, peers, instructor and technology. The learner-related factors were found to be the most important in motivating learners to initiate learning and persist in their engagement with technology. While contextual factors were influential, they did not always play a decisive role in determining learners' behaviours, which differed from previous findings on the significance of personal context (Gao & Krogstie, 2010; Terras & Ramsay, 2012). Peer interaction and instructor support were critical in sustaining learners' efforts and positive experience as they helped address issues in learning with technologies. The Factor Relationship and Interaction model was proposed to explain the complexity of the influences on self-directed mobile learning, contributing a synthesizing model to the fragmented literature on this topic. The study also identified five learner profiles-Tasters, Steadies, Players, Hard-workers and Perfectionists-according to their attitudes, behaviours and engagement patterns with technologies. Learners were found to hold different attitudes and behave differently with the same technologies at different times, producing overlaps and movement between the profiles. These resulted in the Other publications during candidature pertaining to this thesis Refereed Conference Papers • Tran, T. L. N. (2019). Mobile apps and pronunciation training: A self-directed learning approach. In T. Pattison (Ed), IATEFL 2018 Brighton conference selections (pp. 159-161).