This special issue is the outcome of a theme session convened by the above authors entitled “Frictional melting processes and products in geologic materials” held at the 1990 Geological Society of America Annual Meeting in Dallas, Texas, U.S.A. The goals of the session were threefold: to bring together researchers working on various aspects of frictional melting in rock, to provide a forum for the exchange of new ideas and to encourage the production of a thematic set of papers dedicated to the topic. Of the eight papers presented here, the first two (Spray, Swanson) are primarily processand mechanism-oriented. The next (Magloughlin, Maddock, O’Hara) deal primarily with chemical aspects of host-rock-pseudotachylyte relations. The last three mainly provide descriptions of naturally occurring (Koch and Masch, Techmer et al.) and artificially produced (Kennedy and Spray) friction melts. Natural pseudotachylytes are described from strike-slip zones (Magloughlin, Swanson), thrusts and their listric counterparts (Koch and Masch, Techmer et al.) and connecting lateral ramps (O’Hara). Thus, it appears that there exists an association between pseudotachylyte generation and relatively long-lived, largedisplacement faulting and shearing. The prime objective of the meeting was to promote the integration of different approaches