In this chapter and in Chapter 8, which is concerned with multi-degree-of-freedom (MDOF) vibrations, we focus on linear systems. In Chapter 11, the general concept of nonlinear vibration and selected nonlinear systems will be introduced. General references can be found in Schueller and Shinnozuka (1987), Clough and Penzien (1993), Wirsching et al. (2006), Chopra (2003), and Liang et al. (2012).