The world is becoming an increasingly urban place. About 65% of the world's population is expected to live in cities until 2025 (Schell and Ulijaszek, 1999). The excessive urbanization is the source of many problems such as pollution, crime, housing, noise annoyance, congestion, shortages of fresh water and energy, etc. (Tanguay et al., 2010, Van Dijk and Mingshun, 2005). The problem of attaining urban sustainable development is thus an important challenge.
The sustainable development concept was defined by World Commission on Environment and Development as "development that meets the needs of the present, without compromising the ability of future generations to meet their own needs" (WCED, 1987). The necessity of the sustainable development was highlighted at the World Summit of Rio de Janeiro in the summer of 1992, and it was resumed in 2002, at the World Summit in Johannesburg..
Measuring the sustainability in urban areas--which are crucial engines of local socio-economic development, but at the same time present concentration points of environmental decay--is a major challenge for environmental managers and decision-makers. (Moussiopoulos et al., 2010). Sustainable development indicators are a solid base for the regular and long term monitoring of the progress registered in the achievement of strategic objectives of sustainable development and the evaluation of various aspects of sustainability (Hernandez-Moreno and De Hoyos-Martinez, 2010, Ghiga, 2000). They are an indispensable tools for establishing the strategy and policy development, giving a representative image of the three dimensions of sustainable development: society, economy and environment
Many authors (Scipioni et al., 2009) sugest that the adoption of suitable indicators is fundamental to implement sustainable development at the urban level. The use of evaluation indicators and a method for assessing the status of urban sustainable development is required to support urban ecological planning, construction, and management (Li et al., 2010, Hernandez-Moreno, 2010). With sustainability as the goal, the use of indicators for urban monitoring and regulation is becoming more and more in demand (Repetti and Desthieux, 2006).
At international level there are few researches concerning sustainability at city level. Some examples are the studies realized for the cities of Shanghai (Yuan et al., 2003) and Jining (Li et al., 2009) in China, Taipei (Huang et al., 2009) , Granada (Luque-Martinez and Muhoz-Leiva, 2005) and Padua (Scipioni et al., 2009).
In 2008, in Romania, was published the index of sustainable society (Mocanu-Perdichi, 2009), which analyses the current level of sustainable development in eight development regions of Romania. Until present, in Romania hasn't been done such a study at the city level, so the authors have proposed to assess the urban sustainability in four major cities (including the capital of the country) and to rank the cities in terms of sustainability in the period of 2006, 2007 and 2008.
2. THE RESEARCH METHODOLOGY
In the following we analyse the current level of the sustainable development of four Romanian cities highlighting the differences between this cities. The data used for the preparation of this study, are statistically data available at the national and local (municipal) level, which were then interpreted and compared.
For an easy assessment of sustainable development status, we used a set of 18 indicators grouped in four categories, as follows: 4 indicators for the economic dimension, 7 for the social dimension, 5 for the environmental dimension and 2 for natural resources. We have choosed the indicators we considered relevant for Romania.
These 18 indicators are:
1. Gross domestic product per inhabitant (RON);
2. Occupied population per total population (%);