Theories and models on the concept of nursing are reviewed briefly and their contribution then assessed in terms of their potential to advance the scientific basis of nursing practice. The constructs are acknowledged to have moved the focus of nursing from disease to patient need and provided different ways of looking at the nurse's contribution. Yet, despite this advance, the constructs have not been put to the rigours of scientific testing and so remain at the level of speculation. Therefore, towards the developments for the twenty-first century it is proposed that these ideas should be put to the test to establish their relevance in the real world of nursing care. This requires: a framework from which the nature, extent and purpose of care can be specified; independent criteria which can be used to judge effectiveness of care; knowledge as it relates to the different and varied core concepts underlying the practice of nursing; and a working definition of need. In addition, the extent of care for the community needs to be made explicit through research. Together, this represents a challenge to develop theories which are logically related to single core concepts which themselves represent specific areas of actual nursing practice.