This chapter presents an overview of noise reduction methods that have either been implemented or are being developed for future aircraft. It provides a summary from recent noise assessments sponsored by the International Civil Aviation Organization that include noise projections for future aircraft. Components of a turbofan engine that produce noise include the fan, jet, and core. Airframe noise is an ensemble of many different sources distributed across aerodynamic surfaces, appendages and cavities. Trailing edge noise is present for all flow conditions, but is particularly important when an airfoil is generating high lift. Landing gear noise is a dominant source of airframe noise when deployed during approach. When the slats are deployed, there is a slot at the leading edge that emits broadband noise below the wing. Implementation of noise-reduction technologies depends on many factors such as new aircraft and engine design opportunities, requirements, and trades across many objectives such as safety, cost, and performance.