Summary: We investigate properties of propositional modal logic over the class of finite structures. In particular, we show that certain known preservation theorems remain true over this class. We prove that a class of finite models is defined by a first-order sentence and closed under bisimulations if and only if it is definable by a modal formula. We also prove that a class of finite models defined by a modal formula is closed under extensions if and only if it is defined by a \(\diamond\)-modal formula.