Systematic desensitization (SD), developed by Joseph Wolpe in the 1950s, is a form of therapy that attempts to pair two different response systems together, the fear system and relaxation.
By having people do relaxation exercises and then think about gradually increasing anxiety-provoking situations based on their fear hierarchy, Wolpe attempted to have people decrease their anxiety by recognizing that they could remain relaxed even while thinking of the things they feared. By doing this, he was trying to extinguish the fear response to that thought.
SD is the forerunner to ERP therapy, which actually puts people into the situations that they fear in order to have them learn that they can handle the situations and that they do not need to run away from them.
Therapists still use a modified version of SD in the treatment of OCD today when an exposure is hard or impossible to do (for example, if the fear is the death of a loved one, then that would have to be imagined instead of created) or when exposure may bring about such a high level of anxiety that it would be beneficial to imagine it before attempting it.
We can thank Dr. Wolpe for being one of the people who helped to create the behavioral treatment of OCD and anxiety.