In this talk, we will give a survey of the results on unification in modal logic and we will present some of the open problems whose solution will have a great impact on the future of the area. After an introductory part about unification in equational theories, we will consider the case of Boolean unification, we will study the unification problem in Intuitionistic Propositional Logic and transitive normal modal logics like K4, we will introduce the notions of projective and transparent unifiers and we will define filtering unification. Then, we will present the latest results obtained within the context of unification in description logics and in multimodal, tense and epistemic logics.