A Green City is a city that takes responsible political and societal action to meet high environmental quality, which by itself contributes to human well-being (Figure 2.2). So green city is a multidimensional concept that involves economic, environmental, and social aspects. Therefore, Green cities must be an environmentally friendly aspect . Thus the adjective green doesn’t solely refer to environmental problems but also search to integrate social and economic considerations into urban development processes. Over than half of the world’s population now lives in cities, and this share is to grow to 75 % by 2050.
These days the most urbanized regions are Northern America (82 %), Latin America and the Caribbean (80 %), and Europe (73 %) .The green city has the following objectives: clean air and water, the risk of major infectious disease outbreaks in such cities is low, low chemical and physical hazards, pleasant streets and parks and green cities are resilient in the face of natural disasters. Green cities also encourage green behavior, such as the use of public transit, minimize transfers of environmental costs to areas outside the city, make sure progress towards sustainable consumption, and their ecological impacts are relatively small .
The green city designed considerately of environmental impact, inhabited by people dedicated to the reducing of the needed inputs of energy, the waste output of heat, water and food, water pollution and air pollution – CO2, methane. As the meaning of a green city is still new, their development still faces several challenges .Urban sustainability indicators or Green city indicators are tools, which allow city planners, city managers and policymakers to determine the socio-economic and environmental impact like current urban designs, infrastructures, policies, waste disposal systems, pollution, and access to services by residents. They allow for the diagnosis of issues and pressures, and so the identification of areas that might profit from being addressed via good governance and science-based responses. They also allow cities to monitor the success and impact of sustainability interventions .These indicators measure how much a city is a greenness. Currently, 115 indicators across more than 135 cities are being collected each year. The facility is being supported by the World Bank, the Inter-American Development Bank, the University of Toronto, the Government of Canada, and a worldwide network of participating cities .The green city idea is being implemented in more than 100 cities, which uses different indicators. They choose the indicators toward their cities that which indicators can apply in their cities. In Europe, there are many communities that awarded the green city or carry out the green city project, which also uses different indicators. So the green cities indicators differ from region to region.European green capital award is a policy tool of the Commission’s Directorate General for the Environment to promote and improve the quality of urban environments. It’s an annual award that recognizes an outstanding commitment to environmental practices in one European city. Cities are required to have at least 100,000 inhabitants to participate, but there is no upper limit. Every year one European city has been selected as the European Green Capital. The cities are evaluation in a set of 12 environmental indicators which have equal weighting  .The European Environmental Agency has been studied the feasibility of developing an Urban Metabolism indicator system. They gather data from distinctive frameworks that headline 15 indicators for a green city . The Urban Ecosystem Europe (UEE) tool is the result of cooperation between DEXIA, an international banking group, and Ambiente Italia the research consultancy and creator of the tool. The UEE evaluation was being based on a questionnaire that contained 25 indicators . The European Green City Index (EGCI) is a research project conduct by the Economist Intelligence Unit (EIU) and support by Siemens. It’s an assessment of the environmental sustainability of 30 European cities ranging in size from less than 1 million people to more than 3 million people. The EGCI contains 17 quantitative indicators measuring how a city is currently performing, e.g., energy consumption and recycling rate and 13 qualitative indicators assessing cities’ environmental aspirations, e.g., commitments to reduce CO‚‚ emissions or to enhance the share of renewable energy  . The green city indicators are mainly divided into two-part; quantitative and qualitative indicators. Quantitative indicators are being used to describe the three Green City dimensions. In the environmental quality dimension, the largest number of indicators have been found. These indicators allow assessing the environmental performances of cities in eight categories: CO‚‚, Energy, Buildings, Transport, Water, Waste, Air quality, and Green areas and land use. Qualitative indicators are mainly used to assess the city’s environmental policies . It’s seen, which several indicators have been used in every tool. However, the categories present in all tools are Transport, Air quality, and Waste. The CO2, Energy, Water, Green spaces, Building, Acoustic environment, Education, Equity, Safety, Health, and Participation is being used in most of them. So after reviewing, there will be seen that 13 categories of indicators play an important role in evaluating the greenness of a city (Figure 2.3). Here some detail information about these categories are being given, and then indicators of each category have been described in more detail .