In this talk we are going to explain the basic theory of proper actions of groups on $C^*$-algebras as originally introduced by Marc Rieffel in the 80’s and its variations, like the weakly proper actions studied by many authors, including me in a series of papers in collaboration with Siegfried Echterhoff. The results show how powerful the theory of proper actions is and how it interacts with many imprimitivity and duality results associated with group actions and their crossed products.