The purpose of this qualitative study was to look at how teachers use micro-blogging, in this case Twitter (www.twitter.com), for their own personalized professional learning and how effective Twitter is as a professional development (PD) tool. In order to measure the effectiveness of the tool, the researcher first gleaned nine essential characteristics of effective PD from the literature. This list was validated by experts in the PD community. The significance of this study was to reveal how participants actually used Twitter for PD, what their perspectives on the tool were, and how effective their experiences were with Twitter as a PD tool. Results of this study can be used to improve current practice, and provide a low cost, accessible, and available mechanism to foster an on-going, learner-centered, approach to PD, thus allowing teachers to become more involved in their own professional growth. For the 4 participants in this study, Twitter use for PD and its effectiveness varied greatly. The effectiveness of the tool depended on the participant's fluency with the technology and attitude towards social media. For the most fluent participant, Twitter met most of the requirements for effectiveness; however, Twitter use did not automatically provide a mechanism for reflection or self-assessment; nor did Twitter use provide an evaluation of the experience, both requirements of effective PD. With added evaluation and self-assessment processes, and with a fluent practitioner, Twitter does have the potential to be a very effective PD tool with its low cost, accessibility, and availability. [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.]