Cities today are home of more than half of the world’s population. Extended urban conglomerations have grown very fast absorbing the mass influx of rural migrants, especially in low - and middle - income countries.

This phenomenon has contributed to increase the anthropogenic impact on the environment as a result of the many human activities taking place in urbanized areas. Over the years, pressure to sustain growth has increased our consumption of natural resources until the point where we have outstripped our planet’s capability to produce what we consume.

As a consequence, a drastic shift towards more sustainable solutions has become imperative. Different approaches have emerged to solve this problem, among them the concept of urban metabolism. It aims to respond to the pressing request for a more integrating approach that consider cities as a complex system of social and economic factors. In particular, a city’s metabolism is identified by the processes according to which raw materials, food and water are transformed into physical structure, energy and waste. Understanding this cycle has become fundamental for decision makers and policy makers.

The application of the urban metabolism concept for city planning is translated into the quantitative analysis of inputs (energy, food, etc.) and outputs (waste, emissions in the atmosphere). Originally associated to a static model that considered a linearity in a city’s processes - from inputs to outputs – it then evolved to a more cyclical model that aimed at a balanced system where outputs were recycled back as inputs. In both cases, linear and cyclical models, the focus was merely on the human-nature dichotomy which might not be exhaustive enough to cover all situations. In recent years, the discussion has revolved around the need to move to a more complex system, able to consider both biological as well as social and economic factors in order to build a qualitative framework rather than just a quantitative one. Of course, we have to acknowledge that this purpose is much more difficult to pursue.

In a globalised world, collective quantitative data is already a huge challenge since the ancient concept of city space has basically disappeared making it hard to track the flux of inputs and outputs crossing national borders. In addition to that, the need to gather qualitative data that take into consideration social, political, economic factors, too, is even harder. However, a more dynamic framework would allow a deeper understanding of the dependency relations that exist between different countries - especially between developed and developing ones – since some measure to increase sustainable solutions in some parts of the world might negatively affect some others.

As a consequence, practical methods of analysis have to improve in order to provide standards and guideline that can support governments to achieve a more sustainable development.