The moduli space of Higgs bundles over a Riemann surface has the structure of a completely integrable Hamiltonian system now known as the Hitchin system. The critical locus consists of the points at which the Hamiltonian vector fields become linearly independent. There is a well established general concept of nondegeneracy for such points and under those conditions one obtains integrable systems of lower dimension. The talk will first focus on applying these methods to the above system and then discuss topological aspects of the moduli space which are realized by critical loci.