Traditionally, models of instructional leadership espouse data-informed decision making in response to student assessment outcomes as one of the core school leader behaviors. In recent years, rising expectations from accountability policies and related assessment practices have myriad implications for school districts, specifically in the areas of standards-driven reform, student assessment systems, and professional development models. As a result, demands on schools to collect and use student assessment data to inform curricular and instructional decisions has expanded. While principals are typically held responsible for school improvement efforts, more contemporary models of instructional leadership incorporate teachers as classroom-based leaders of assessment practices in forums such as professional learning communities. School and classroom assessment leaders engage in behaviors such as (a) identifying an assessment vision, (b) fostering group goals, (c) providing a model of data-informed decision making, (d) promoting teacher job-embedded professional learning experiences, (e) evaluating instructional practices with specific feedback, and (f) strategically aligning resources to school improvement goals. Unfortunately, school districts face many challenges with assessment leadership due to barriers in beliefs about assessments, time with and access to tools and training, and knowledge and skills about how to operationalize effective assessment practices that yield positive student outcomes. The purpose of this study was to explore assessment leadership as a construct among P-12 school and classroom leaders in one large district in Florida. Data were collected using an Internet-based survey constructed from existing qualitative and quantitative measures of key components of assessment leadership established in the literature. A series of descriptive and inferential analyses were conducted to (a) explore the factor structure of the instrument and (b) evaluate the influence of assessment learning experiences, beliefs, and knowledge on assessment practices. Relationships among variables were examined when considering moderating variables for school role (i.e., school-level administrator or classroom teacher as professional learning communities facilitator) and school type (elementary or secondary). Limitations were discussed to inform future research in this critical area of school improvement. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]