This paper considers some of the reasons why motivated students in suitable learning environments may fail to learn from competent teachers. It draws upon work in the psychology of learning, and the considerable body of research that has been undertaken to explore the nature and origin of learners' alternative conceptions in science. A synthesis of ideas from this previous work suggests a simple typology of 'learning impediments' in terms of the mismatch between the learner's cognitive structure and the teacher's expectations. It is suggested that this classification system may be a useful tool that, alongside techniques to probe prior knowledge, can help teachers diagnose and overcome such impediments to intended learning, and thus make teaching more effective. Although deriving from research into the learning of science, it is suggested that the typology can be applied to conceptual learning across the curriculum.