An expansion joint is a device used to allow movement in a piping system while containing pressure and the medium running through it.

Expansion Joints, besides acting as sections of a pipe, capable of withstanding the design temperatures, pressures, and media, also provide the flexibility that makes them necessary. That flexibility can be either axial, lateral or angular . However, in many installations, some rigidity or structural strength is needed to support piping loads or control the motion of the bellows and the pipe. Thus, features, like Pipe, Flange, Limit Rods, Tie Rods, Internal Sleeve (Liner), Shroud (External Cover) , are then added to the expansion joint.

Various calculations are used to determine the theoretical life expectancy for a given design. EJMA (Expansion Joint Manufactures Association) is the predominant standard used. The expansion joint manufacturers who make up EJMA are today's leading authorities on expansion joint design.