We consider a class of reaction-cross diffusion systems naturally arising in Population Dynamics. In those systems, cross diffusion terms appear only in one of the two equations (triangular case). We present results of existence of weak solutions for these systems. The solutions are obtained as the limit of the solutions of a microscopic model where only standard diffusions appear. The results use Lyapounov-like functionals and duality lemmas. This is a joint work with Laurent Desvillettes.