Seals are required to fulfill critical needs in meeting the ever‐increasing system performance requirements of modern machinery. This chapter provides design insight and application for both static and dynamic seals. Static seals reviewed include gaskets, O‐rings, and selected packings. The chapter provides useful insight to designers approaching an O‐ring seal design, including the basic sealing mechanism, preload, temperature effects, common materials, and chemical compatibility with a range of working fluids. Dynamic seals reviewed include mechanical face, labyrinth, honeycomb, and brush seals. Honeycomb seals are used extensively in mating contact with labyrinth knife edges machined onto the rotor in applications where there are significant shaft movements. After brazing the honeycomb material to the case, the inner diameter is machined to seal tolerance requirements. For each of these seals, typical configurations, materials, and applications are covered. Where applicable, seal flow models are presented.