This thesis is concerned with the actual process of nursing in two major and distinctive cardiothoracic units in Scotland. In order to understand the nursing process the research has been designed to investigate nurses' perceptions of real patients. A methodology has been developed in order to analyse nurses' perceptions of patients whilst actively engaged in care. Personal Construct Theory (Kelly 1955) is proposed as providing an appropriate conceptual framework for the investigation. The elicitation of nurses' perceptions in the Pilot study yielded a set of common perceptual constructs which were then standardised and tested in the Main Study. From interviews with nurses who care for the same patients but in four distinctive clinical settings: the operating theatre, the recovery room, the surgical ward and the intensvie care unit, a data-set was obtained which on analysis indicated that specific environmental, medical, social and psychological criteria influenced nurses' perceptions and the nursing process. The conclusions drawn from the study have implications for the development of the nursing role, the delivery of care and the analysis of the nursing process in general.