This document discusses the response of 1-degree-of-freedom systems to harmonic excitation. It covers: 1) The response of an undamped system, which has a steady state solution that is the sum of the homogeneous and particular solutions. The maximum amplitude is proportional to the force amplitude divided by the spring stiffness. 2) The response of a damped system, where the maximum amplitude depends on the frequency ratio and damping ratio. It is highest at resonance when the forcing frequency matches the natural frequency. 3) Applications to damped systems subjected to harmonic base motion, rotating unbalance, or other periodic excitations. The ratio of response amplitude to static deflection is called the magnification or amplification factor