The aim of the article is to point out to the specificity and difficulties an expert psychologist faces while producing a court expertise in family and guardianship cases. Such a diagnosis is prepared at the request of a family court. The questions asked by the court in family and guardianship cases determine the range and aim of diagnosis including the type of examined case. Having considered the court’s questions, a psychologist formulates hypotheses and operationalizes variables. This article will present the main areas of problems which arise while developing opinions in family and guardianship cases. The two main issues will be discussed: the range of an expert psychologist’s competences in the light of the court’s expectations frequently exceeding an expert’s capabilities and the impact of the choice of research methods on the quality of an opinion issued to obtain the final ruling in the court case.