Services are quintessential in the current economical landscape. Enterprises and businesses at large rely on the consumption and providing of services to ensure their operations and to realize their business offers. That is, nowadays businesses all over the world are interconnected with each other by complex service-centric webs called service networks. The ubiquity and pervasiveness of service networks call for models, methods, mechanisms and tools to understand them and harness their potential. This paper investigates the modelling of the service networks with a focus on business relationships and exchanges of software services among the involved parties. The contribution of this work is threefold. Firstly, we provide an overview of what service networks modelling can offer in combination with Business Process Management (BPM) and Service Oriented Architecture (SOA) technologies. Secondly, we propose a formalism to model service networks that depicts them as aggregations of participants - e.g. enterprises or individuals - that offer, request, consume and provide services to each other. With the goal of providing a foundation for the alignment between service network- and business process models, we finally map the constructs of our service networks modelling formalism to the ones of the Business Process Modelling Notation (BPMN).