Following Chevalley, a subgroup is called a Cartan subgroup if it is maximal nilpotent and each of its normal subgroups of finite index is almost seflnormalizing (in the ambient group). I will describe Cartan subgroups in groups definable in o-minimal structures, a model-theoretic context which incorporates semialgebraic groups over real closed fields, many real Lie groups, and algebraic groups over algebraically closed fields of characteristic 0.We will notably see that there are finitely many conjugacy classes of Cartan subgroups, and that they cover the ambient group very largely.