The literature on services is replete with references to service eco-systems, yet no attempt has been made to develop a set of principled conceptual underpinnings for these. This chapter aims to address this gap. This chapter also seeks to design the formal basis for practical tools to support the design and maintenance of service eco-systems. It describes a high-level Business Service Modeling Language (BSRL) that is general enough to support the modeling the full spectrum of services, spanning from web services on the one extreme to abstractly defined business services on the other. Based on this language, it describes a taxonomy of relationships that might hold between services in an eco-system. These relationships are then leveraged to formally define a notion of equilibrium in a service eco-system. The chapter then extends the analysis to a deeper level of detail, by considering inter-operation relationships between the process designs that realize services. The chapter briefly considers the challenge of service-oriented analysis and design, and in particular, addresses the question of which combination of functionalities might be packaged as a service, thus leading to the set of inter-related components constituting a service eco-system.