This chapter from Volume 1 of "Research-Based Reform of University Physics" describes  Matter & Interactions, a modern calculus-based introductory curriculum that emphasizes the power of fundamental principles, and guides students through the process of starting from these principles in analyzing physical systems, on both the macroscopic and the microscopic level. The continual emphasis on the application of fundamental principles and on the atomic nature of matter makes possible the integration of topics that are traditionally taught as disconnected: mechanics and thermal physics are intertwined, and electrostatics and circuits are analyzed using the same tools for both topics. The development of the curriculum has been shaped by research on learning and on research in physics education. For additional information, see http://www.matterandinteractions.org/.