In the past 150 years nursing science has developed into a discipline of its own which has integrated features of various stages of theory development. So far most nursing theories, originally borrowed from other disciplines, have been adapted and moulded to suit the nursing context. The theories developed after Nightingale, such as developmental, need-oriented, interaction, system-oriented and simultaneity theories have come about this way. They have depicted the areas of interest in nursing which has thus been defined as the domain of main concepts and their interconnections. The methodological characteristics of research in science are derived on the basis of ontological and epistemological principles. The domain of nursing science is assumed to be an integral whole which is not divided into parts even for the purpose of research. Scientific data are obtained from this undivided world by means of man's human signification. In research, this undivided world is approached from the phenomenological tradition of science: particular topics are thematized as something, while the other aspects of the phenomenon are excluded. The accumulating scientific data are integrated to form a holistic body of knowledge which helps in understanding the phenomena in greater depth and from different perspectives. The historical development of nursing science reveals problems and conflicts which are derived from the tradition of empirical science of the last few decades. Research solutions following the empiricist tradition offer only limited opportunities to focus on the central research objects of nursing. Therefore nursing research should proceed from the phenomenological basis and follow solutions made on the ontological level. Recognition of this solution helps in defining and clarifying nursing research.