This report describes a comprehensive knowledge synthesis of professional learning communities (PLCs) and their impact on science, technology, engineering, and mathematics (STEM) teaching and learning in K-12 education. This report is organized around the following sections: Background, Overview of Key Findings, Methodology, Results, and Discussion and Implications. Three overarching questions guide this secondary analysis: (1) What do individuals know about PLCs involving STEM teachers?; (2) How robust is the research base about PLCs involving STEM teachers?; and (3) What further research and development are needed? Appended are: (1) References; (2) List of Advisory Board Members; (3) List of Expert Panelists; (4) Expanded Methodology; (5) List of Organizations Searched for PLC Statements; and (6) Acronyms Used in Citations. (Contains 7 tables, 4 figures and 1 footnote.) [For related report, "STEM Teachers in Professional Learning Communities: From Good Teachers to Great Teaching," see ED521328.]