The article describes the approach used in the psychiatric assessment of a person's competence to testify and discusses different disorders. Psychiatrists are often asked to comment on the competence to give evidence if witnesses' behaviour suggests psychopathology or they are undergoing psychiatric treatment or psychotherapy. Competence to testify is assessed in two steps. Impaired capacity to give evidence does not necessarily have to be considered unless a psychiatric diagnosis has been made. It is very rare for a witness to be declared incompetent to testify; incompetence is assumed only in the context of classic psychiatric illnesses, psychotic states or intoxications. Other diagnoses generally do not justify the assumption of incompetence to testify.