Cartridge hydraulic valves have been used in mobile and industrial applications for many years with continued interest by end users for cavity standards. Effort has been directed by NFPA and ISO to generate standards. There are two basic types of cartridge cavities, slip-in and screw-in. An ISO standard has been generated for slip-in cavities. The screw-in cavities are essentially in two configurations: O-ring boss (rigid construction), and guided thread (floating construction). There is a need to review the concepts of the cavities and their advantages, as manufacturers of equipment seek to improve products to meet customers' needs.