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Quality concerns, often referred to as nonfunctional requirements, service-level agreements, quality attributes, performance constraints, or architecturally significant requirements, describe system-level attributes such as security, performance, reliability, and maintainability. In conjunction with functional requirements, these quality concerns drive and constrain a system's architectural design and often introduce significant trade-offs that must be carefully considered and balanced. The dependencies that exist between requirements and architecture have been referred to as the twin peaks of requirements and architecture. The guest editors of this special issue describe this unique situation.