This thesis explores the nature of Self-Directed Professional Development (SDPD) pursued by university English as a Foreign Language (EFL) teachers in Saudi Arabia with a special emphasis on investigating the impact of one model of SDPD ‘Virtual Community of Practice’ (VCoP) through WhatsApp on teaching practice. Due to the exploratory nature of this study, an interpretive epistemological stance was adopted informing two designs in two stages. In the first stage, a sequential mixed-methods design with two phases was implemented, starting with a quantitative phase employing online questionnaires, followed by a qualitative phase using semi-structured interviews to gain a comprehensive understanding of the nature of SDPD at the macro-level of the EFL context in public universities, in terms of its models, type of activities and reasons behind pursuing this form, as well as the challenges that inhibit teachers form practising it. A total of 260 participants from 13 Saudi universities completed the questionnaire and 14 of them participated in interviews. Quantitative data were analysed through SPSS to get descriptive data, and qualitative data were analysed thematically. In the second stage, a holistic case-study design was adopted to gain deep insights into the micro-level of SDPD through investigating the nature and impact of one model of SDPD, ‘a self-directed VCoP’, via WhatsApp on EFL university teachers’ teaching practice in one Saudi university. Participant online observation was carried out for six months followed by semi- structured interviews with nine participants. Qualitative data from both methods were analysed thematically. The findings of the mixed-methods stage revealed that although EFL university teachers employed both individual and collaborative SDPD, they tended to favour individual rather than collaborative activities. The findings also showed that SDPD enhances teachers’ autonomy, self-empowerment, motivation, confidence and professional identity. Moreover, key reasons (e.g. personal, academic, administrative) for pursuing SDPD were identified, as well as the challenges and limitations of both types of offline and online SDPD. The findings of the case study showed that self-directed VCoP via WhatsApp positively impacted on teachers’ practice, themselves and their students. Most importantly, using an informal medium of communication e.g. the first language of teachers, ‘Arabic’, demonstrated a significant impact on EFL teachers’ learning and communication. Revealing the effectiveness of SDPD as an alternative approach to teacher’s PD, the thesis concludes by indicating a need to devote equal attention to SDPD (as both a term and a concept) as an aspect of the formally acknowledged form of in-service PD. It also makes a number of significant recommendations, and highlights implications, to improve the policy and practice of SDPD for policymakers, teachers and researchers.