As shown in Tables 1.2 and 1.3, we note that a large number of materials are used for various components in nuclear power systems. It is important to state here that relatively large structures can only be fabricated using welded joints and the designers need to account for the varied properties of the dif­ferent materials and their welds; often welds are known to be more sensi­tive to radiation and corrosive environments. As pointed out by Roberts,3 in many cases, nuclear grade materials are fabricated to more stringent speci­fications than those for other technologies and are subjected to inspection and surveillance following in-reactor exposures. Stress states experienced by different components vary depending on locations, for example biax­ial stresses in thin-walled cladding tubes and more complex ones in pipes, elbows and their welds; a time dependent constant load leads to creep failure in out-of-core structural components while in-core materials experience irra­diation enhanced damage thereby further shortening their life. All structures, especially massive ones such as reactor pressure vessels (RPVs), invariably contain flaws and cracks that need to be taken into consideration through fracture mechanics and structural integrity analyses. It is therefore necessary to develop appropriate constitutive laws and models taking into account the individual or combined effect of: (i) instantaneous elastic and plastic defor­mation, (ii) time dependent recoverable deformation (anelastic strain), (iii) time dependent plastic deformation (creep), (iv) strain accumulation due to cyclic loading (fatigue), (v) corrosive environment effects, (vi) compositional effects such as dynamic strain ageing leading to premature failures and finally (vi) radiation damage and effects. The common ageing-related degradation mechanisms are described in the subsections which follow while more details are given in various chapters of the book — Part I covers major phenomena and Part II pertains to specific structural components and varied NSSSs.