Pseudo-differential operators, PsDOp, are natural extensions of differential operators, DOp; they have many essential properties in common with DOp, but they do not - in general - have the locality property. An operator L acting on functions u(x) is called "local" if the value of Lu at a point x depends only on the values of the function u in the neighborhood of this point. While PsDOp do not — in general — have this property, they share most other properties with differential operators.