Abstract-We present a research agenda on an emerging topic in software engineering, namely the synergy between Software Product Line Engineering (SPLE) and Service-Oriented Computing (SOC). Our proposal is to develop rigorous modelling techniques as well as analysis and verification support tools for assisting organisations to plan, optimise, and control the quality of software service provision, both at design time and at run time. We foresee a flexible engineering methodology according to which software service line organisations can develop novel classes of service-oriented applications that can easily be adapted to customer requirements as well as to changes in the context in which, and while, they execute. By superposing variability mechanisms on current languages for service engineering, based on policies and strategies defined by service providers, we envision the possibility of identifying variability points that can be triggered at run time to increase adaptability and optimise the (re)use of resources.