This paper introduces an approach to the specification of system families. The main ingredient of this approach is the definition of use cases hand in hand with a requirements object model. Instead of specifying individual systems, we specify a domain, i.e., a conceptual space of possible systems, in which individual systems can be defined by fixing a number of variation points. In that way we obtain a strong cohesion within the family and concise specifications of its members. We also describe a process suitable for this specification approach and indicate how the transition to the design phase can take place. Our approach was validated in one large project and several smaller ones.