The recent availability of data about cities and urban systems opens the exciting possibility of a new Science of Cities'. Urban morphology and morphogenesis, activity and residence location choice, mobility, urban sprawling and the evolution of urban networks are just a few of the important processes that can be discussed now from a quantitative point of view.

In this talk, I will illustrate on various examples how the combination of data, physical arguments and modeling help in understanding urban systems. I will start with some socio-economical indicators that can describe various aspects of cities such as the total energy spent in transportation, the total length of the street network, the total delay due to congestion, etc.

The variation of these quantities with population usually reveals interesting scaling laws with non trivial exponents that call for an explanation. I will show here how a physicist approach can help in solving these puzzles.