This paper will highlight the evolution of defensible space theory - from Newman's original theoretical model to some of the subsequent theoretical and empirical developments that have been made in the past 35 years. By charting these developments in our understanding of the underlying mechanisms of defensible space, the aim of this paper is to illuminate the aspects of the theory that remain ambiguous and those that have been clarified to some extent by developments in criminological research. This paper will suggest that the most ambiguous of Newman's concepts is that of 'milieu'. It will be argued that this key defensible space concept draws on situational aspects of spatial layout and accessibility, land-use patterns and routine activities of place. With this in mind, this paper will attempt to re-conceptualize defensible space within the context of situational crime prevention theory by elucidating the effect that routine activities of place have on territoriality and the creation of defensible space.