The basic theory of linear systems is developed in a unified and accessible manner, with parallel, independent treatment of continuous-time and discrete-time linear systems. Modest mathematical background is assumed. There are many examples to help the reader, and carefully chosen exercises. The presentation has been repeatedly class tested and refined. This edition includes ten new chapters on discrete-time, time-varying linear systems, the most complete, basic treatment available. It also includes expanded treatment of the time-invariant case, new material on time-varying, reduced-dimension observers, geometric treatment of time-invariant canonical structure theorem, and many improvements to the continuous-time presentation.