This publication presents the proceedings of ICPMSE-4, the fourth international conference on Protection of Materials and Structures from the Low Earth Orbit Space Environment, held in Toronto April 23-24, 1998. The conference was hosted and organized by Integrity Testing Laboratory Inc. (ITL), and held at the University ofTorontos Institute for Aerospace Studies (UTIAS). Twenty two industrial companies, six universities and fourteen government agencies from Canada, USA, United Kingdom, France, Israel, Russia, Ukraine and the Netherlands were represented by over 75 participants indicating increasing international co-operation in this critical arena of protection of materials in space. Twenty-seven speakers, world experts in their fields, delivered talks on a wide variety of topics on various aspects of material protection in space. Representatives from the Canadian, American, European and Israeli space agencies as well as from leading space research laboratories ofmajor aerospace industries gathered at UTIAS to discuss the latest developments in the field of material and structure protection from the harsh space environment.