GRAPPLE is an EU funded IST FP7 project that brings together a group of researchers into adaptive learning technology and environments and developers of learning management systems (LMSs), in order to offer adaptive learning as a standard feature of future LMSs. This papers presents the overall architecture of GRAPPLE, and explains some of the main challenges in creating a truly general-purpose adaptive learning environment (ALE) that can be used with different LMSs and a distributed architecture of user modeling services. In particular the paper describes the conceptual adaptation model, its translation into adaptation rules, and some technological challenge that results from separating the concerns of adaptation and user modeling.