The Helmholtz equation models time-harmonic wave motion phenomena and is consequently one of the most important equation in mathematical physics. For infinite domains, its mathematical analysis is rather difficult due to a default of coercivity of the associated operator: the solutions of the Helmholtz equation are not of finite energy and are not uniquely defined. Many authors have developed a theory to bypass these difficulties. The limiting amplitude technique, the absorbing principle and the unique continuation theorem are, to my opinion, the main ingredients of this theory. In this lecture, I will give an introduction to these three techniques.