I will motivate a quick introduction to pseudo-differential operators by discussing the so-called Calderon's problem, that is, whether knowledge of the Dirichlet-to-Neumann map (or the Neumann-to-Dirichlet map) at the boundary  determines the conductivity of a body in the interior. This problem has applications in Electric Impedance Tomography (EIT). I will in particular discuss uniqueness at the boundary, interior uniqueness using Complex Geometrical Optics (CGO solutions), and if time permits, layer stripping reconstruction algorithms in the case of isotropic conductivities.