This book explains the case of a 19-year-old patient who, in the last six months, has experienced intense and irrational fear of entering elevators in different contexts. In the intervention treatment, the systematic desensitization technique is used, which allows the patient to learn to expose himself to the stimuli that generate aversion, and in this case it is the use of the elevator. The systematic desensitization technique includes the process of training in progressive muscle relaxation, establishing a hierarchy with the situations that generate anxiety, training in imagination through a gradual exposure to anxiogenic situations and live exposure to unpleasant situations. The intervention process is developed in ten sessions. The research is applied and corresponds to the Univariate Multiconditional design of the Applied Behavior Analysis (ABA). The results indicate that in the pre-treatment baseline, fourteen problem behaviors were obtained; compared to the post-treatment baseline measurement, where problem behaviors did not occur.