An exemplary case, a school case, a beautiful case, a scenario, an extreme case, a particular case, a thorny case, a borderline case, etc. The details of the case study are manifold and express the fact that any reasoning, explanation, theory once or another about the need to explore and deepen the properties of an observable singularity. Publishing a book on the single case issue is part of the reflection on scientific methods in psychology. It is also about working towards a constructive repositioning of this way of doing science, giving the floor to those who specialise in these issues or who are directly concerned by them through their research or practice.