This book offers a self-contained, elementary and yet rigorous treatment of linear system theory and optimal control theory. Fundamental topics within this area are considered, first in the continuous-time and then in the discrete-time setting. Both time-varying and time-invariant cases are investigated. The approach is quite standard but a number of new results are also included, as are some brief applications. It provides a firm basis for further study and should be useful to all those interested in the rapidly developing subjects of systems engineering, optimal control theory and signal processing.