We present a logical framework that is able to deal with variability in product family descriptions. The temporal logic MHML is based on the classical Hennessy--Milner logic with Until and we interpret it over Modal Transition Systems (MTSs). MTSs extend the classical notion of Labelled Transition Systems by distinguishing possible (may) and required (must) transitions: these two types of transitions are useful to describe variability in behavioural descriptions of product families. This leads to a novel deontic interpretation of the classical modal and temporal operators, which allows the expression of both constraints over the products of a family and constraints over their behaviour in a single logical framework. Finally, we sketch model-checking algorithms to verify MHML formulae as well as a way to derive correct products from a product family description.