In this chapter, we will talk about the testing phase, which is the final phase of a design thinking (DT) process, as illustrated in Figure 7.1. In it, we will use prototypes to study how the people for whom we develop the solution interact with them. This way, we can reveal new solutions to existing problems or find out if the solutions we develop were really successful or not. The results generated from these tests are used to redefine one or more of the problems established in the previous phases of the process, and to understand in a clear and complete way any unforeseen situation that could appear when interacting with a product or service in a real environment.