The Next Generation Science Standards (NGSS) are composed of three intertwined dimensions--disciplinary core ideas, science and engineering practices, and crosscutting concepts--that provide a foundation for what students should know and be able to do at various grade levels. The eight science practices outlined in the NGSS are critical components of scientific sense making, but currently there is limited guidance on what these practices should "look like" in K-12 classrooms. Based on an extensive literature review and a multi-round modified-Delphi panel of expert practitioners, Horizon Research, Inc. (HRI) developed a primer to unpack the science practices. The primer outlines key elements of each practice--what students should be doing when engaging with each science practice--and provide illustrative examples of how these key elements might play out in classrooms across different grade bands and subject areas.