Historically, educational policymakers have focused on and invested heavily in accountability testing of "learning" to leverage improvement in student learning. Through accountability testing, policy makers aim to communicate standards, establish performance goals, provide data through which educators can analyze and improve school programs and student performance, and establish incentives and sanctions to motivate action. Today, however, there is growing recognition of the limitations of accountability testing of learning, and wide acknowledgment and accumulating evidence of the crucial role that formative assessment--"assessment for learning"--can play in helping all students achieve rigorous standards. This paper pairs an in-depth exploration of current formative assessment practices and research with guidance on application of the presented conceptual model to the Next Generation Science Standards (NGSS). The paper includes a selective review of diverse literature focused on effective formative assessment practice, and uses this literature to describe and justify a proposed model. The paper highlights special considerations for, and provides examples of the model's application in Next Generation Science Standards. Implications for the design of coherent assessment systems that support student learning are included.