The fundamental principle of the Common Law is that witnesses can only testify about what they saw, heard or felt. They are not allowed to expound any deductions based on the facts. However, in cases in which specialized knowledge is required, expert witnesses are called to present their opinions based on scientific facts. However, judges cannot check the credentials of persons testifying as experts. As a result, bogus experts may mislead the court. Furthermore, with many branches of knowledge, such as sociology, psychology and psychiatry, the facts are often hard to identify, a formulation of principles poses many difficulties, and opinions are often not reliable enough for judicial purposes. For example, in homicide cases, when the verdict depends on the assessment of the defendant's mental state at the time of the crime, expert opinion is often based on interviews with the defendant and the defendant's family. The medical opioion depends on how critical the expert's attitude is toward facts learned from such interviews. Another problem may arise when distinctions are not drawn between the normal and pathological response of the defendant to some kinds of stimuli. The reactions of persons who are not suffering from any mental abnormality are for the jury to assess, and in such cases the expert testimony should not be needed. In spite of problems, psychologists can make a useful contribution to the judicial process when their methods are used to reveal latent traits such as sadism and propensity to delinquency. The evidential frontier between what is permissible and what is not for an expert should be better defined, and information about expert witnesses should be sought from professional societies. References are made to French and U.S. law.