Acritalia has performed a preliminary definition of the Internal Multiscreen Insulation (IMI) which forms part of the external Thermal Protection System (TPS) of the Hermes space plane. The purpose of the insulation is to protect the space plane ‘Cold structure' from the high heat loads experienced during the vehicle reentry. The IMI is a flexible insulation and its design concept makes use of reflective foils separated by low density felted fibre materials. It is normally located between the external (rigid) shingles and the fuselage, but will also be used to prevent excessive heating of those zones behind the vehicle's ‘Hot Structure' (nose cap and wing leading edges). The IMI blanket capability must be such that, while withstanding a limit temperature of 1300° C and an ultimate temperature of 1450° C, it will maintain the cold structure below 200° C. The design life is 15 years or 30 flights. The minimization of mass is also a major constraint.This paper deals with the extensive activities of materials research and testing carried out in order to identify candidate screen/felt materials and to verify their suitability for use in the high temperature oxidizing environment of the reentry. The design philosophy together with the detailed thermal mathematical model, where the variable properties of screens, felts and air are taken into account, to optimize the insulation design, is also described. Mass budgets are presented for several typical zones of the vehicle. Finally the results of thermal conductivity tests are given.