The homotopy groups of the inverse limit of a tower of fibrations

We carefully present an elementary proof of the well known theorem that each homotopy group (or, in degree zero, pointed set) of the inverse limit of a tower of fibrations maps naturally onto the inverse limit of the homotopy groups (or, in degree zero, pointed sets) of the spaces in the tower, with kernel naturally isomorphic to ${\lim }^1$ of the tower of homotopy groups of one dimension higher.