Abstract: Compact and Normal Operators are two of the best understood classes of operators in terms of existence of invariant subspaces. Nevertheless, it is still open the Halmos Problem: Does every compact perturbation of a normal operator have a non-trivial invariant subspace? In this talk, we will motivate the interest of this particular problem and will focus on the 'baby' case: rank-one perturbations of diagonal operators. We will also introduce some basic aspects from Local Spectral Theory and will show the relation of this theory with invariant subspaces, that will play a prominent role on > the study of these particular operators.