In this essay, the advantages and disadvantages of single case and group study research design methodologies are discussed. The arguments supporting one or the other study design are introduced relying on most prominent scientific opinions in the field. The terms of single and double dissociation are introduced. Examples from relevant neuropsychological studies are provided in order to illustrate the use of case and group study methodologies for single and double dissociation. The two study designs are shown to deal with single and double dissociations in their own way and to contribute significantly to the advancement of neuropsychological sciences. It is concluded that the research design method selected should be tailored to deal with the specific problems raised by the nature of the question under research and fit in the realm of the chosen cognitive theory.