This is a sequential explanatory mixed-methods study of teachers' professional development experiences within two Educational Service Agencies in the northeastern part of the United States. It examines how much and what types of professional development teachers receive, and the factors that contribute to school leaders' decisions for offering professional development and teachers' decisions for participating in professional development. The study was based on what the field knows about the characteristics of effective professional development, and the lack of comparison between those traits and what we know about current professional development practices. The study is composed of multiple phases, which include the analysis of a secondary data set provided by the two Educational Service Agencies. The data set provides four years of data regarding professional development offerings in two regions of the state and was analyzed for trends and patterns related to the types and amounts of offerings. This data informed a subsequent phase in which I interviewed 28 school leaders and teachers about their professional development experiences and decision-making. I used a deductive coding approach to review the data and develop my findings. The findings are presented as a descriptive narrative of the data, augmented with tables and figures. Several themes emerged from the data that helped answer four questions: 1) How much and in what types of professional development do teachers participate? 2) What factors influence the professional development that is offered to teachers? 3) What factors influence the professional development that teachers choose to take? 4) How do school leaders and teachers talk about professional development? A key idea emerges that a universally-accepted definition or typology of professional development does not exist. Implications for future research, regulations and policy, and professional development facilitators', school leaders', and teachers' practice are discussed. [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.]