There is now compelling evidence that humans are altering virtually all of the Earth’s ecosystems. Vitousek et al. (1997) noted that more than half of the Earth’s fresh water is used by humans, nearly half of the land surface has been transformed by human action, more atmospheric nitrogen is fixed by human activities than by all natural terrestrial processes combined, and human activities are leading to significant losses of biodiversity. As a consequence of these actions, most (if not all) ecosystems can arguably be considered human-dominated ecosystems, regardless of whether humans actually occupy (live within) them. However, humans are also creating new ecosystems specifically for dwelling: these are urban ecosystems (Steams and Montag, 1974). These new, synthetic ecosystems are unquestionably human-dominated, and yet urban ecosystems are both qualitatively and quantitatively different from other human-dominated ecosystems in terms of development, sphere of influence, and potential impacts. Understanding these differences hinges on our ability to distinguish between urban and human-dominated. In 1900, only 9% of the world’s human population lived in “urban environments” (so called by the World Bank, 1984). This figure had increased to 40% by 1980, 50% by 2000, and is expected to increase to over 66% by 2025 (World Bank, 1984; Simpson, 1993; Rodick, 1995; Brockerhoff, 1996). The increasing abundance of such environments has not gone unnoticed by ecologists. Indeed, the “urban landscape is a well-appreciated form of landscape” (Weddle, 1986) and offers numerous areas of scientific study since urban landscapes are often characterized by the presence of exotic flora and fauna, an imbalance between biotic immigration and extinction rates (Rebele, 1994), and the presence of pollution in air, water, and soil (Botkin and Beveridge, 1997). Until recently, however, relatively little ecological research was conducted in urban settings. Indeed, ecologists have primarily sought to understand their subjects of study in the absence of humans and have usually considered humans chiefly as agents of disturbance (Pickett and McDonnell, 1993; Costanza, 1996). A recent survey of research papers in the foremost ecological journals between 1993-1997 revealed that only 25 of 6157 papers (0.4%) dealt specifically with urban species or were carried out in an urban setting (Collins et at., in review). Ecologists have recently begun to recognize this oversight, however, calling for more research into how overall ecosystem structure and function shape and are shaped by urban development (Matson, 1990; McDonnell and Pickett, 1990; Botkin and Beveridge, 1997; Walbridge, 1997; Parlange, 1998). Such a pursuit is not as easy as it sounds, however. There are logistical problems incurred from working in urban settings, such as difficulty in obtaining permission to conduct large-scale experiments on private property, as well as vandalism to field equipment (see also Yalden, 1980). Similar problems are faced in most ecological studies. What is different about conducting research on how patterns of urbanization affect ecological processes, though, is in determining what constitutes an “urban ecosystem.” By this, we mean recognition that an urban area is not simply a human-dominated area. Fundamentally, a landscape defined as urban shows some effects of human influence. Taken Iiterally, this could mean that the most remote sites could be called urban simply because humans have influenced a portion of their area at some point in time (e.g., by the presence of a secluded vacation cabin or even aboriginal ruins). Clearly, this description of urban is too broad to be very useful, and it confounds the differences between human-dominated and truly urban ecosystems. There is thus an evident need to remove the uncertainty with which ecologists define urban ecosystems and to correct oversights regarding definitions ( or lack thereot) of what it means to be “urban.” In this paper, we will examine definitions of the term “urban” as used by ecologists and look towards the social sciences for guidance in creating a more quantitative description of what an urban ecosystem is. In recognizing that urbanization is both an ecological and a social phenomenon (thereby recognizing that urban ecology is an interdisciplinary field), we will compare and contrast definitions of “urban” as used by ecologists and social scientists and will provide some case studies of how interdisciplinary studies have defined and used “urban.” This process will illuminate both strengths and gaps in current descriptions of urban systems. It will also provide both ecologists and social scientists with guidelines to quantify the urban setting of their own research projects. Although we recognize that no single definition of “urban” is necessarily more correct than another, we will demonstrate that demographic, economic, political, perceptive, and cultural criteria, when used in conjunction with geophysical and biological criteria, provide a more complete and useful definition.