Reliability is critical because it contributes to a system’s warfighting effectiveness as well as its suitability in terms of logistics burden and the cost to fix failures. For each system, there is a level of basic reliability that must be achieved for the system to be militarily useful, given the intended Concept of Operations (CONOPS). Reliability is also one of the most critical elements in determining the logistics infrastructure and footprint. Consequently, system reliability should be a primary focus during design (along with system technical performance, functions, and capabilities). The primary objective is to achieve the necessary probability of mission success and minimize the risk of failure within defined availability, cost, schedule, weight, power, and volume constraints. 
Definition: Reliability is the ability of a system or component to perform its required functions under stated conditions for a specified period of time. 
Guide: DoD Reliability Availability and Maintainability (RAM)
Reliability requirements are included in the Initial Capabilities Document (ICD) and Capabilities Development Document (CDD) and flowed down to the system specification. When developing reliability requirements as part of a Key Performance Parameter (KPP) and Key System Attributes (KSA), options that should be considered and implemented to enhance system reliability and achieve the Reliability KPP/KSA include:
- Over-designing to allow a safety margin
- Redundancy and/or automatic re-configuration upon failure allowing graceful degradation
- Fail-safe features (e.g., in the event of a failure, systems revert to a safe mode or state to avoid additional damage and secondary failures). Features include real-time reprogrammable software, or rerouting of mission-critical functions during a mission
- Calibration requirements
- Reliability Growth Program
Reliability estimates evolve over time
Generally, the initial estimates are based on parametric analyses and analogies with like or similar systems operating in the same environment and adjusted via engineering analysis. As the design evolves and as hardware is prototyped and developed, the engineering analysis becomes more detailed. In addition to estimates and modeling, testing at the component, subsystem, or system level may be necessary to assess or improve reliability. Approaches such as accelerated life testing, environmental stress screening, and formal reliability development/growth testing, should be considered and incorporated into program planning as necessary. To assure the delivery of a system that will achieve the level of reliability demanded in-field use, a methodical approach to reliability assessment and improvement should be a part of every well-engineered system development effort. 
Reliability Main References
- The DoD Reliability, Availability, and Maintainability (RAM) Guide provides a structure, references, and resources to aid in implementing a sound strategy.
- The Reliability, Availability, Maintainability & Cost (RAM-C) Rationale Report Manual provides guidance in how to develop and document realistic sustainment KPP and KSA requirements with their related supporting rationale; measure and test the requirements, and manage the processes to ensure key stakeholders are involved when developing the sustainment requirements.
- It is crucial the reliability approach be planned to produce high confidence the system has been developed with some margin beyond the minimum (threshold) reliability.
- The Reliability and Maintainability requirements work together
AcqLinks and References:
-  Website: Wikipedia – Reliability
-  Defense Acquisition Guide (DAG) – Chapter 5
- DoD Reliability, Availability and Maintainability (RAM) Guide
- The Reliability, Availability, Maintainability & Cost (RAM-C) Rationale Report Manual
- Reliability – A look at Four Decades of Reform within the DoD by Larry Crow PhD
- Website: Military Directives, Handbooks and Standards Related to Reliability
- Website: IEEE Reliability Society