We start with this chapter the second part of the book which is devoted to electrical systems, and in particular to DC-to-DC power converters. The study of these devices constitutes an active area of research and development in both power electronics and control theory. Switched DC-to-DC converters have an ubiquitous variety of industrial and laboratory applications thanks to their reduced cost, simplicity and off-the-shelf availability. This part of the book consists of two chapters. In view of the presence of the switches some new considerations with respect to those made in Chapter 2 and Appendix B, are needed to formalize the mathematical modeling. This is done in Chapter 6, which is fully devoted to modeling and the exploration of the structural properties useful for PBC, which as we will see later, applies verbatim for this class of systems. This material is presented in Chapter 7.