In the past nursing has used a medically oriented perspective; consciously or not, nursing practice, education and research have been guided by the same conceptual frame of reference as has medicine. For nursing to justify its claim to being an independent health profession offering a particular service to society, it must adopt its own conceptual base, one that indicates those phenomena that are of concern to nursing and those health problems that nursing must try to solve. Many nurses have already chosen to base their teaching, research and nursing care on one of the existing conceptual models for nursing. The challenge for the 21st century is that all nurses adopt an explicit conceptual base. Broader than a theory, a conceptual model specifies nursing's focus of inquiry and may thus lead to the development of theories which will prove useful not only to nurses but to other health professionals as well. Since nursing exists to provide a necessary service to mankind, its conceptual base must be evaluated by using specific social criteria.