This document discusses principles for high-quality science learning in afterschool programs. It outlines five key principles: learning that is active, collaborative, meaningful, supports mastery, and expands horizons. Each principle is explained and related to how science learning aligns with it. For example, science is an active endeavor involving hands-on experiences, it is collaborative involving teamwork and sharing ideas, and it allows students to expand their horizons through discovery and new experiences. Additional topics covered include the importance of adult mentors and applying these principles to design improved STEM activities.