The aggregation equation is a nonlinear and nonlocal PDE used to model the dynamics of individuals in interaction (chemotaxis, crowd motion, ...). In this talk, I will try to give some insight about the dynamics of this model, then I will present some convergence results of the relaxation approximation à la Jin-Xin for the aggregation and discuss some numerical aspects. This is a joint work with Benoît Fabrèges, Frédéric Lagoutière et Nicolas Vauchelet.