“…An alternative to these classical numerical methods is the development of some techniques for the projection of the PDEs onto a low dimensional subspace. In accordance to these techniques, the original PDEs are transformed into a set of ordinary differential equations (ODEs) known as reduced order model (Aksikas et al, 2007;Americano da Costa Filho et al, 2009;Bouaziz & Dochain, 1993;Christofides, 2001;Dochain et al, 1992;Hoo & Zheng, 2001;Petre et al, 2007;Shvarstman et al, 2000). As a result, the first objective of this chapter is to provide the mathematical tools, which are used for most of numerical methods, for solving PDEs and, on this basis, to give a brief outline of the most commonly employed techniques.…”