Book excerpt: This primer addresses the basic concepts of model-based systems engineering. It covers the Model, Language, Behavior, Process, Architecture, and Verification and Validation. It is a call to consider the foundational principles behind those concepts. It is not designed to present novel insights into MBSE so much as to provide a guided tour of the touchstones of systems design. It is a guide to the new MBSE acolyte and a reminder to the experienced practitioner. It is our hope that you find this primer valuable. We welcome your comments and suggestions about improving it. Much of what we have learned about how it should be organized and presented has come from thoughtful contributions from the readers of the 1st edition.