Most current military and all civil engines are operated under 'safe life' procedures for their critical components. Experience has shown that this philosophy presents two drawbacks: The move towards designs allowing higher operational stresses, and the use of advanced high-strength alloy make it likely that a disc burst could happen (following a rapid crack growth) well before the statistically-based 'safe life' has been achieved. It is potentially wasteful of expensive components, since it has been estimated that over 80% of engines discs have ten or more low cycle fatigue lives remaining when discarded under 'safe life' rules. Damage tolerance begins an alternative life being philosophy, the Sub-Committee on 'Damage Tolerance Concepts for the Design of Engine Constituents' has therefore decided to conduct a series of four Workshops addressing the areas critical to Damage Tolerance design of engine parts. The present report includes the papers presented during Workshop II dealing with Defects and Quantitative Materials Behaviour. It also includes the content of the discussions which followed the presentations. Keywords: Endurance general; Fatigue mechanics; Engine disks; Cracks; Gas turbines; Nato; Alloys; Metals; Aircraft engines. (AW).