Barriers are defined as discontinuties in the broadly defined marginal costs of communications, usually leading to a discontinuity in interaction. This paper addresses conceptual issues about barriers to communication. Several typologies of barriers are given. For example, according to the degree of permeability, barriers may discourage interaction, or may even be entirely impermeable, but examples may also be given of barriers which stimulate communication in one or both directions. Another typology of barriers addresses reasons of existence, including natural barriers, barriers created for protection purposes, barriers created for reasons of convenience, etc. The paper concludes with a number of suggestions for further research on communication barriers.