In the following, we collect some basic material well known from classical potential theory in the Euclidean space R3. First we have a closer look at the already mentioned fundamental solution of the Laplace operator. Observ-

ing its specific properties, we are able to formulate the third interior Green formula. Mean value theorems and a maximum/minimum principle are the canonical consequences. Harmonic functions are recognized to be analytic in their harmonicity domain. The Kelvin transform enables us to study harmonic functions that are regular at infinity. Keeping the regularity at infinity in mind, we are finally led to exterior Green formulas. The third exterior Green formula is formulated analogously to its interior counterpart, thereby observing the regularity at infinity.