The ballistic performance,and behaviour,of an armour system is governed by two major sets of variables,geometrical and material,Of these,the consistency of performance,especially against small arms ammunition,will depend upon the consistency of the properties of the constituent materials.In a body armour system for example,fibre diameter,areal density of woven fabric,and bulk density of ceramic are examples of critical parameters and monitoring such parameters will form the backbone of associated quality control procedures.What is often overlooked,because it can fall into the User\'s domain,are the interfaces that exist between the various products;the carrier,the Soft Armour Insert (SAI),and the one or two hard armour plates (HAP1 and HAP2).This is especially true if the various products are sourced from different suppliers.There are between 30 and 150 individual layers within a typical body armour system,and each of the interfaces between each of those layers will,in some way or another,contribute to the ballistic perfor-mance of the system.For example,consider the following interfaces/interlayers:(i) the frictional,sliding,inter-ply surfaces within a soft armour pack,and also between the pack and the carrier,(ii) the air-gaps that may develop within the soft armour pack,(iii) the interconnecting space between the soft armour pack and the hard armour plate,(iv) the nature of the interfaces between adjacent plies of a multiplied backing laminate,even in a highly compressed Ultra High Molecular Weight Polyethylene (UHMWPE)variant,(v) the interlayer between the ceramic and its substrate,within a HAP,and (vi) the geometrical fit between two hard armour plates within a stacked body armour system.This paper will provide a User-friendly overview of all such interfaces and provide unique guidance as to their criticality and influence.