Many concerns for the state of the environment first developed in urban areas where the changes in environmental conditions began to affect human health. As part of ecological systems, cities affect and are affected by natural cycles. They import water, energy and materials which are transformed into goods and services and ultimately returned to the environment in the form of emissions and waste. Their high concentration of people and activities make cities major contributors to local, regional and global environmental change. On the other hand it is the same concentration of people that provides unique opportunities for economies of scale and resource conservation.

Urban systems can be described according to their population, societal relationships and economic activities. The urban form structure and landuse are other important variables which affect the quality of the urban environment and the impact of urban settlements on the regional and global environment. In terms of systems ecology, the city can be compared to an ecosystem with its own structure, functions and metabolism. 

Changes in the quality of the urban environment are rooted in the patterns of urban activities. Urban density, mobility and lifestyles are reflected in the demand for space and the flow of resources. Concentration of people and activities in a limited space places a heavy demand on local natural habitats.