This chapter is concerned with the response of single-degree-of-freedom systems subject to harmonic excitation. It introduces the complex exponential representation of harmonic forces to discuss phenomena such as resonance and beating in undamped systems. The focus then shifts to damped systems, highlighting key differences from the undamped case. The frequency response function is presented, with an emphasis on interpreting frequency response curves. Matlab is integrated throughout the chapter to enhance understanding and visualization. Five illustrative examples are provided to reinforce the concepts discussed, including two simple experiments that readers can easily replicate to qualitatively test the theory.