A model, derived from the literature, is presented of the influence of learners' preconceptions on the amount of mental effort invested in learning from television. The proposed model suggests that learners' preconceptions are influenced by the characteristics of the media, the characteristics of the task, and the characteristics of the learner. The theoretical and research basis of these proposed relationships is reviewed. In general, research on learners' preconceptions of television and video indicates that learners perceive television as an easy medium, believe that it requires little mental effort, and think that they would learn little from a video-based lesson. However, their preconceptions may change as they become more experienced. Learners appear to make a decision about the difficulty of the task, decide how much effort to exert in processing the lesson, and modify their effort expenditures if characteristics of the lesson differ from those expected. Practical implications of the proposed model for instructional design are considered, and needs for further research are outlined. (Contains 36 references.) (SLD)