[en] This review of failure development is based largely on a literature survey up by some personal interpretation and judgement where conflicting evidence appears to exist. A brief review of known causes of defects in LWR fuel rods is included. Primary defects in fuel rods may not be single, isolated, leak sites, PCI defects being a classical example where very many penetrating cracks can form in a single fuel rod. The only secondary defect mechanism is internal hydriding, although knock-on events can follow on the same, or neighbouring rods, from defects that cause and interference with local coolant flow. The major part of this survey comprises information on leak rates from typical defects, causes and rates of development of primary leak sites, meachanisms and rates of development of secondary hydride defects and analysis of activity release measurements in an attempt to characterise the number and severity of defects present