ABSTRACT
This chapter presents some general considerations on the steps that are needed for building a cosmological model. It considers simple cosmological models containing ordinary fluids like matter and radiation drawing a general strategy to build a cosmological model and derive all its properties. The equation of state corresponds to specifying the nature of the fluid filling the universe and is, therefore, a very important link between cosmology and fundamental physics. This means that general relativity alone is not sufficient to specify the cosmological model. The existence of the initial singularity signals that going back in time, the validity of Friedmann cosmology breaks down at some point, before reaching the singularity and it cannot give any description on what happens too close to the singularity or before.
