The main subject of this chapter is the analytical presentation of the most important techniques, concepts, and methods related to the description of continuous-time systems in the complex frequency domain. The chapter begins with the definition of the fundamental concept of a system transfer function as the Laplace transform of the impulse response of a linear time-invariant (LTI) system, as well as a description of the concepts of poles and zeros of first- and higher-order systems. The basic theory associated with the stability of continuous-time systems is presented, together with the pole-zero cancellation technique and the stability criteria of Ruth and Hurwitz. Expressions for the step and frequency response of the system are then derived, and the technique of geometric estimation of the transfer function and the frequency response function (which is defined as the transfer function estimated on the imaginary axis) from the positions of the poles and zeros of these functions on the complex plane is illustrated via appropriate examples. The well-known Bode diagrams are introduced and constructed for specific transfer functions, together with the presentation of several techniques for describing continuous-time LTI systems by means of block diagrams and signal flow diagrams via Mason’s rule. The chapter concludes with a description of continuous-time systems in the state space for the case of the complex frequency domain, namely, the derivation and solution of dynamic equations using the Laplace transform, as well as the expression of the transfer function as a function of the four fundamental tables used in this type of representation.