The interrelationships between energy and civilization was proposed in the late 19th century [see Martinez-Alier, J., 1987. Ecological Economics. Blackwell, Oxford]. Geddes [Geddes, P., 1915. City in Evolution. Williams and Norgate Ltd., London] also proposed the concept of linking energy flow with urban development in the early 20th century. Many recent studies of the relationship between urban development and energy have been limited to the use of fossil fuels; energy theories have seldom been used to study the evolution of urban systems. The purpose of this paper is to help develop theories of urban energetics and contribute to the understanding of the mechanisms of urban development. We use the concepts from systems ecology [Odum, H.T., 1983. Systems Ecology. Wiley, New York] and emergy theory [Odum, H.T., 1996. Environmental Accounting: Emergy and Environmental Decision Making. Wiley, New York] to relate the driving energies of urban systems to their structure, economy and organization. Hypotheses of urban energetics including changes in the diversity of emergy sources, changes in urban metabolism, energy hierarchy, and relation between emergy and money flows are proposed and tested based on past research results on Taiwan cities (Huang, S.-L., Odum, H.T., Chang, S.-L., 1996. Ecological energetic evolution of urban system, A Report to the Chiang Ching-Kuo International Scholar Exchange Foundation (RG008-D-’92). Graduate Institute of Urban Planning, National Chung-Hsing University, Taipei, Taiwan (in Chinese); Huang, S.-L., 1998a. Urban ecosystems, energetic hierarchies, and ecological economics of Taipei metropolis. J. Environ. Manage. 52, 39–51; Huang, S.-L., Lai, H.-Y., Lee, C.-L., 2001. Energy hierarchy and urban landscape system. Landscape Urban Plan. 53, 145–161]. Thereafter a system model of urban ecosystem is developed to investigate the relationships between energy flows and urban development. The preliminary findings of this research indicate that intensive and diversified emergy sources build up the structure and enhance metabolism in urban areas. Four mechanisms of urban energetics are also addressed as a result of simulations: energy convergence; urban-subsidized environmental flows; external-dependency system; maximum power system.