A mixed-method research design was used to identify the components of professional development in two high achieving, high poverty K-12 Missouri public school districts, that most contributed to improved student achievement. A rigorous quantitative site selection process that incorporated five variables was developed and used. Seventy-eight percent of teachers at one district and 61% of teachers at the other district participated in an online survey that collected their perceptions on their district’s professional development. Sixteen teachers at one district and 18 at the other district were purposively sampled and interviewed to develop a graphical model of their respective professional development process and to gain additional insight into their professional development processes that they perceived contributed to improved student achievement. Six conclusions were drawn from a combination of quantifiable data sources and the sum of teacher perceptions as systematically interpreted from all qualitative data sources. The study conclusions focused on (a) collaboration as a key professional development process, (b) leadership as an important element of effective professional development, (c) the relationship of professional development to student achievement, (d) the importance of technology and curriculum alignment, (e) evaluation as an essential element of improved teaching and learning, and (f) the continuous acquisition of new learning and skills development.