  Graded $C^*$-algebras by a semi lattice were introduced and studied by Anne
Boutet de Monvel, Vladimir Georgescu and their collaborators in relation with
the quantum N body problem. This thesis is devoted to a systematic study of
these algebras and their properties. In particular, we show that they are
completely described by their homogenous subalgebras and that they are
invariant under several operations such as tensor products, crossed products
(by actions of locally compact groups that respect the graduation). We give the
$K$-theory of graded $C^*$-algebras and finally, we study some examples of such
algebras.
