This document presents a research synthesis for the "Educator Assessment Literacy Professional Learning Screener," a screening tool developed to help state, district, or school leaders select a high-quality assessment literacy professional learning service by evaluating the quality of content and implementation plans. The purpose of this document is to provide details regarding the research supporting development of the screener and directing interested readers to references for more information. The development of the screening tool was guided by two research questions focusing on assessment literacy content specifications for teachers and leaders, as well as implementation features of professional learning programs: (1) What is the foundational content knowledge and skills K-12 educators (teachers and leaders) need to be considered 'assessment literate' based on the research literature and experts in the field? and (2) What is best practice in implementation of in-service K-12 educator professional development according to reviews of the body of recent literature?