The class of media characteristics, which is generic to them and which may be of potentially great relevance to learning, is the way in which media select, highlight, structure, and present information, i.e., their "languages" or symbol systems. How, if at all, and why do symbol systems, in general, differentially relate to cognition and learning? First, they address themselves to different aspects of the content conveyed through them. Second, they are processed by different cognitive systems or apparati; hence, given, a particular learner and a particular task, some require more mental effort to process than others. Third, symbol systems vary as to the mental skills they require. Finally, media's symbol systems, which are internalizable and used as tools of thought, can cultivate mental skills. Three functions are attributed to codes that qualify them as skill-cultivators: activating skills, short-circuiting skills, and supplanting skills. If media's symbol systems can indeed affect, or be made to affect, the mastery of mental skills, then using and studying media as convenient delivery systems miss perhaps their greatest potentials and dangers. (VT)