This action research study was designed to elicit urban, secondary school teachers' understandings of themselves as adult learners and their perceptions of job-embedded professional development in a single, urban school district in Connecticut. The conceptual framework that guided this study was derived from Knowles, Holton, and Swanson's (2011) andragogy in practice model because it offers a systematic framework that consists of core adult learning principles that are applicable to professional learning situations. Knowles et al.'s (2011) model encompasses three components: (a) the goals and purposes for adult learning; (b) individual and situational differences for adult learners; and (c) andragogy, which entails six core principles of adult learning that are the foundation of this study. The six core principles of adult learning (i.e., andragogy) are the learner's: (a) need to know; (b) self-concept; (c) prior experience; (d) readiness to learn; (e) orientation to learning; and (f) motivation to learn. Two data collection strategies were used to answer the research questions: an Internet-based survey and an in-person interview. The survey was researcher-developed and designed to align with Knowles et al.'s (2011) six core principles, literature related to effective professional development strategies (Trivette, Dunst, Hamby, & O'Herin, 2009), and job embedded professional development (Croft, Coggshall, Dolan, Powers, & Killion, 2010). The interview guide, following Flanagan's (1954) Critical Incident Technique (CIT) framework, was designed to complement the survey and elicit rich examples of teachers' professional development experiences. A convenience sample of 289 urban, secondary school teachers were recruited to participate in this study. Of the 289 survey respondents, 32 elected to participate in the in-person interview phase of the study. Procedures associated with quantitative and qualitative research were used to analyze the data, which consisted of survey data and verbatim transcripts of in-person interviews. This generated 71 significant findings about urban secondary school teachers' understandings of themselves as adult learners and their perceptions of their professional development experiences. Conclusions were drawn and recommendations for practice and future research are presented. Study participants perceived that they learned best when core principles of adult learning (Knowles et al., 2011) were applied during their professional development experiences. Yet, they only occasionally or rarely experienced adult learning principles in their professional development experiences in their school district. As well, study participants found professional development sessions that incorporated evidence-based professional development strategies (active learning, role playing, modeling) and formats (professional learning communities, peer coaching) to be the most effective. Yet, they reported that they only occasionally or rarely experienced these strategies and formats in their professional development experiences in their school district. [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.]