The aim of this article is to provide expert witnesses, especially psychiatrists, other physicians, as well as psychologists, with basic information on logic, rhetoric and eristic, useful in their professional practice. The reason is that these skills, undoubtedly belonging to the classical education, are not standard elements of teaching professionals in the fields mentioned above. Having the expert knowledge, ability to diagnose, to conduct a therapy and to prognosticate does not indicate the ability to conduct effective arguments. This work is based on Cardijin’s method (See – Judge – Act). It sometimes happens that a well-prepared expert opinion (psychiatric and psychological) is discredited for non-substantive reasons due to some eristic and rhetorical tricks. Having such experiences, some expert witnesses resign from giving opinions. To help avoid such situations, this work presents the most important definitions of logic, rhetoric and eristic. Examples of propositional calculus, selected models of reasoning, rhetorical figures and eristic tricks can be used in presenting professional expertise. These examples are accompanied by propositions of responses to arguments used by persons willing to discredit expert witnesses’ opinions. Furthermore, this work offers a scheme of answering questions and doubts of the parties in court hearings.