As mentioned in Chapter 1, this book addresses classical numerical techniques to solve various vibroacoustics problems. In practice, a typical problem involves a structural domain coupled to bounded or unbounded fluid domains and sound-absorbing materials. We then seek to predict the coupled vibratory/acoustic response of the whole system subjected to the given excitations and boundary conditions. In this chapter, we introduce the fundamental equations governing the linear sound wave propagation in fluid domains (acoustics), in solid domains (elastodynamics), and in porous sound-absorbing materials (poroelasticity). We also establish the coupling equations between these domains. All these partial differential equations will be then solved using specific numerical methods, which will be presented in Chapters 3 and 4.