Common core standards and individual education plans specify learning goals for students. But when a student's performance falls short of these goals, it can be challenging - even for professionals with a range of training and disciplinary expertise - to determine the cause of the student's struggle and to plan the necessary supports. Individualized learning solutions are necessary, because one-size-fits-all interventions don't work. This landmark guide presents an evidence-based approach to assessment and instruction in K-12 education that takes into account individual differences in students. The guide identifies the developmental skills to be assessed and taught in early childhood, middle childhood, and adolescence, and it provides principles for tailoring assessment and intervention to individual students, who exhibit sizable developmental, individual, cultural, and language differences. Importantly, it also explains how to facilitate communication and collaboration among interdisciplinary teams in education - professionals who are legally required to work together yet have such different training and disciplinary expertise that they seem to speak different languages. This timely, comprehensive resource will help local schools and districts meet not only narrow legal mandates but also broader best practices for helping students learn. A special companion website provides additional resources and recommended readings.