I will explain a new construction that attaches a $C^*$-algebra to a Grothendieck topos satisfying some topological conditions (a sort of local compactness). In fact one will have both a reduced and a maximal $C^*$-algebra, a Banach ``L1" algebra, and an algebra of compactly supported functions exactly as in the case of topological groupoids. I will also give a brief overview of how Grothendieck toposes relate to some other objects of interest for this workshop (groupoids, inverse semi-groups, quantales...) and how the construction discussed above recovers a lot of examples of classical constructions of $C^*$-algebras: all $C^*$-algebras of \'etale groupoids, graph $C^*$-algebras and their generalizations, inverse semi-groups $C^*$-algebras and a large portion of general topological groupoids, convolution $C^*$-algebras, etc.