The "Next Generation Science Standards" (NGSS Lead States 2013) lay out an ambitious agenda for students to simultaneously engage with disciplinary core ideas, crosscutting concepts, and scientific practices. This does not just mean new expectations for teaching; it also requires new formative assessments. In contrast to summative assessment, used to "evaluate" student learning at the end of an instructional unit, formative assessment is something that teachers and students can do together, every day, to "monitor" student learning and provide timely feedback. Formative assessment also helps teachers modify and fine-tune their teaching practices. Developing intriguing questions and activities for formative assessment and providing useful feedback to students are challenging. Fortunately, the authors' research has shown that teachers can design this ambitious type of assessment with the support of their colleagues. In this article, the authors present the research-based, four-step Formative Assessment Design Cycle (FADC), which guides science teachers through developing, enacting, and revising formative assessments. In the FADC, a group of teachers design common assessments together and then use them in their own classrooms, meeting later to discuss the ideas students shared and what they plan to do next. Each element of the FADC was informed by the results of two research projects involving science teachers from four high schools over several years. These studies found that teachers using the FADC develop better formative assessments, synchronize their units, and learn more about science and student thinking; likewise, student learning increases the more teachers use the cycle to design their formative assessments.