In this chapter, it is argued that research involving adaptive educational hypermedia will be advanced by attention to two main areas: (a) the articulation of principled design features for adaptive hypermedia systems and (b) rigorous research documenting the learning efficacy of particular design approaches for different domains and learner groups. As an example of design and research in these two areas, a case study of a program of hypermedia research related to the knowledge mediator framework (KMF) is provided. First, a discussion of non-adaptive KMF hypermedia design elements and learning tasks is provided, followed by a short overview of the research findings from studies involving the use of different KMF systems. Next, current efforts are discussed to create adaptive KMF hypermedia using a learning agent module that employs semantic assessment and learner modeling in order to provide adaptive content and adaptive learner scaffolding. A general consideration of theory, research, and methodological issues related to current work in the field of adaptive educational hypermedia is also provided.