I will describe how to model the properties of non-ideal fluids using effective, soft, many body potentials. Although they are not derived from a microscopic systematic procedure, it is possible to understand their properties from a systematic saddle point expansion. Such an approach makes it possible to derive their equilibrium thermodynamic and structural features, opening the possibility to relate the effective parameters characterizing the soft potentials with their collective properties. I will also describe how such many body potentials can be used to analyze the properties of complex fluids out of equilibrium.