This paper discusses the service broker, a function introduced into next-generation networks to manage interactions among applications, to reuse existing applications in a combined fashion, and/or to enable existing applications with capabilities such as presence, location, and policy. With the service broker, a minimal set of applications can be configured in a multiplicity of ways as its elements are brought into play in mix and match arrangements. For the degree of flexibility needed for the service broker to support unique service combinations, the service broker must be programmable. Various functional subcomponents enable these service broker capabilities. These subcomponents include service descriptors, the mechanisms to identify the logic that governs how the applications interact; user and endpoint data managers, the entities that present user-specific and endpoint-specific information; and session contexts, the transient entities that contain the context associated with an instance of call/session or multi-call/session. The session context includes state information and provides multi-session awareness that allows both simultaneous and sequential state-dependent sequential activity to be managed. This paper introduces a lightweight, programmable, Session Initiation Protocol (SlP)-centric service broker architecture and the concept of the “steplet,” which is central to this architecture.