In Product Lines Engineering many studies are focused on the research of the best behavioural model useful to describe a product family and to reason about properties of the family itself. In addition the model must allow to describe in a simple way different types of variability, needed to characterize several products of the family. Modal Transition System (MTS) is one of the more relevant behavioural model that has been broadly studied in literature and several extensions have been also described to more finely address behavioural variability. Furthermore MTS and its extensions define a concept of refinement which represents a step of design process, namely a step where some allowed requirements are discarded and other ones become necessary. In this chapter we present a bunch of variants of the classic MTS definition with the associated refinement definitions and a discussion on their expressive power.