<p>Since some twenty years the literature on scaling laws is booming, especially regarding their applications to urban systems. Is this a real breakthrough in the understanding of city dynamics, or just a passing fad for a new scientific fashion? What are the results of this research trend? Can we draw interesting directions from it to deepen theoretical knowledge and to improve urban policies? These questions can be addressed to most of models and theories considered as participating in the “new science of cities”. Indeed, complex systems share a few structural and dynamic properties that incite considering they could share theoretical fundamentals as well. However it is time to throw a critical and reflexive doubt on these claims. The question is not about rejecting the instruments leading to formalization, but to replace them in the chain of intellectual and societal elaboration of science. For instance, the conception of scaling laws evolved as they were confronted with a variety of empirical data and a diversity of systems of cities. By comparing the results obtained with those of other modelling approaches, it is possible to make a provisional assessment of the contribution of this work to the evolution of urban theories and models. Some basic principles explain the great difficulty of transferring and integrating models between the natural/experimental and social sciences. These issues need to be widely discussed in order to consolidate theories on urban systems and avoid too risky interventions in the dynamics of complex systems before new models can be proposed to planners.</p>