Implications of research into human ability and metacognition for adaptive instruction are examined. Traditionally, adaptive instruction has largely involved the adaptation of instruction to suit the cognitive idiosyncrasies of individual learners. Through the development of learning strategies and metacognitive abilities to manage them, this paper discusses how adaptive instruction should aim at fully interactive adaptation between the learner and the instruction. Studies reviewed equate mental effort with depth of processing, and suggest that little will be learned, in terms of schema change, unless mental effort is invested by the learner. For adaptive instruction to be truly effective, learners must be challenged, but not to the point where they no longer believe they can succeed. Conclusions are drawn concerning the relative effectiveness of adapting instruction to learners and learners to instruction. Implications for the design of adaptive instruction are presented. (PN)