Autonomy is regarded as an essential element of professional status. Especially nurse managers have an important role in searching for ways to empower nurses and to provide conditions that promote autonomous practice. This article provides an overview of the empirical research literature on the professional autonomy of nurses in hospital settings. The aim is to clarify the concept and to give information on how the autonomy of nurses has been studied through the decades. The definitions of autonomy and the results of empirical research are reviewed. Further, the article offers a description of the methodologies used in the studies of the autonomy of nurses over the years: data collection methods, samples and research contexts. The review focuses on empirical articles published between 1966 and December 2009 retrieved from the CINAHL and MEDLINE databases. The studies were classified based on a content analysis. This review draws attention to the complexity of the concept of autonomy and its various definitions. Moreover, the data for studies have been collected primarily by questionnaires; the sampling methods have been nonrandomized and the samples varying. It would be necessary to study autonomy in different countries using the same instrument.