This introduction to linear system theory has been class tested at several universities. It focuses on time-varying linear systems, with frequent specialization to time-invariant case. Optional chapters pursue refinements and extensions, many confined to time-invariant linear systems. This text offers a clear, careful theoretical treatment, as well as modular organization for flexibility and provides compact, basic treatments of esoteric topics such as the polynomial fraction description and the geometric theory. There are also results that have not previously appeared in text form, for example, realization theory for time-varying linear systems, noninteracting control for time-varying linear systems and output feedback stabilization for time-varying linear systems. This text is intended for graduate level courses in linear system theory, theory of time-varying linear systems and linear state equations.