This chapter provides the context in which the book is situated. After a brief history of the development of slow-wave transmission lines throughout the 20th century, the general concept of slow-wave transmission lines is introduced, with a theoretical approach defining the slow-wave factor based on the analysis of the volumes occupied by the electric and magnetic fields and their corresponding energies. Then, the three types of slow-wave transmission lines used today are briefly described: slow-wave coplanar waveguide (CPW), slow-wave microstrip, and slow-wave substrate integrated waveguide (SIW), highlighting their particular advantages. Finally, the three key points justifying the interest in slow-wave transmission lines are clearly explained: performance, miniaturization, and degrees of design freedom. The latter is rarely mentioned, yet it proves to be important in the context of increasingly complex technologies.