In this paper we study the admissible resules of intermediate logics. We establish some general results on extention of models and sets of formulas. These general results are then employed to provide a basis for the admissible rules of the Gabbay-de Jongh logics and to show that these logics have finitary unification type. The admissible rules of a logic are precisely these rules under which the set of its theorems is closed. These rules arise naturally from a logic, though they may not be plainly.