This session focuses on developing understanding around geometric thinking and big ideas in HS Geometry. Participants will engage in developing a deeper understanding of the importance of spatial sense, identifying characteristics of van Hiele’s levels, participate in constructing definitions from diagrams, and discuss how proofs are an integral part of mathematics. The math content in this PD is aligned to the ACT Aspire, primarily the reporting categories of Geometry, Modeling, and Justification and Explanation. THIS SESSION IS A PREREQUISITE FOR ALL OTHER GEOMETRY CONTENT SESSIONS.