Abstract. The aim of this monograph is to present a comprehensive survey of results about existence, multiplicity, perturbation from symmetry and concentration phenomena for the quasi-linear elliptic equation nX i,j=1 Dx j (aij(x, u)Dx i u) + 1 2 nX i,j=1 Duaij(x, u)Dx i uDx j u = g(x, u) in Ω, where Ω is a smooth domain of Rn, n ≥ 1. Under natural assumptions on th