A rigorous K-12 science education builds student knowledge in core scientific content and equips students to analyze problems, use evidence, and investigate solutions. The education requirements of careers across the economy are increasing and all students--regardless of whether they pursue science as a career--need greater proficiency in the knowledge and practice of science to be fully ready for college and the workforce. A coalition of states joined forces from 2011 to 2013 to develop a new set of science standards, the Next Generation Science Standards (NGSS), which outline what it means to be fully college and career ready in science. These standards are internationally benchmarked and draw from the practices of the highest-performing countries on international assessments, such as Singapore, Finland, Korea, Canada, and Japan. The NGSS are structured so that students are asked to engage in hands-on practices as they learn subject-based content; through an emphasis on cross-cutting concepts, the standards also build students' knowledge of ideas that apply across science disciplines. The standards follow a clear developmental progression so that knowledge builds logically from kindergarten through twelfth grade. The Framework for K-12 Science Education, developed by the National Research Council and published in July 2011, forms the basis for the structure and scientific content of the NGSS. The Framework presents three key dimensions that are incorporated in each of the science standards: (1) Practices; (2) Cross-Cutting Concepts; and (3) Disciplinary Core Ideas. Twenty-six states voluntarily joined the process to develop science standards based on the Framework. As of March 2014, 11 states and the District of Columbia have officially adopted the NGSS, and each state is now developing its own implementation strategy. The case studies in this document provide a closer look at the strategies four states utilized through the NGSS writing and review process. The first two states, Rhode Island and California, have officially adopted the standards and are currently in the process of developing implementation plans. Minnesota and North Carolina were involved as lead states in the NGSS development process and provide useful examples of how state leaders are paving the way for a more rigorous K-12 science education.