Athanasios Papoulis' classic text was the first to present digital techniques as an integral part of a unified course in system theory and design, rather than as a separate unit. The enduring success of Circuits and Systems undoubtedly is due in large part to the author's concentration on fundamental ideas explained in the context of simple illustrations. The text develops analog systems parallel to digital systems, emphasizes the concepts of linearity, superposition, impulse response, frequency response, and system function. Laplace transforms and z-transforms are treated briefly, but completely, and the introduction to digital and sampled-analog simulation is based on the approximation of the convolution integral by a sum. The development of the material as a deductive discipline strengthens the student's analytical ability in the engineering course.