This International Standard describes early reliability assessment methods for items based on field data and test data for components and modules. It is applicable to mission, safety and business critical, high integrity and complex items. It contains information on why early reliability estimates are required and how and where the assessment would be used. Finally, it details methods for reliability assessment and the data required to support the assessment. To estimate durability (life time or wear-out), the physics-of-failure method is used. Three types of assessment are discussed in detail: – the similarity approach; – models for durability analysis; – handbook methods. Clause 6 provides an introduction to reliability assessment and Clause 7 the management of the process. Clause 8 describes the data needs, sources and types for assessments and Clause 9 provides details of the assessment methods. Annexes A and B provide additional information to aid understanding of the similarity analysis and durability analysis. This standard is applicable to making reliability estimates for specifications, design, design modification and support engineering.