Questionnaires quantify formal aspects of unidirectional communication. This definition indicates the major limitations of research based on such techniques: it will be unbalanced and restrictive, and therefore incomplete. The wide use of questionnaires is often unavoidable in medical and scientific research, yet they are often developed with insufficient attention to the needs that they are intended to meet. Their design may be imperfect even in relatively straightforward circumstances such as the reporting of laboratory observations, but inappropriate as well as imperfect in others, such as the study of individual quality of life. This concept and others related to it are defined, together with some major sources of error in questionnaire development and use. Steps that can be taken to improve the situation are also mentioned.