Abstract The ASME B31.12 Code covers piping in gaseous and liquid hydrogen service and pipelines in gaseous hydrogen service. This chapter is based on the 2014 edition of B31.12, which includes three major Parts: General Requirements (GR), Industrial Piping (IP), and Pipelines (PL). It also covers Mandatory Appendices I through XI and Nonmandatory Appendices A through F. After a brief background and coverage of the scope, intent, and organization of the Code, the chapter examines Part GR, which covers selection of materials to resist deterioration in hydrogen service. It lists the common requirements for welding, brazing, heat treating, forming and testing of welded and brazed joints, qualification of the welding and brazing procedures, and qualification of the welders and brazers to those procedures. Part GR provides information on inspection, examination, and testing along with operating and maintenance procedures that affect the safety of hydrogen transmission and distribution facilities. It also provides requirements for the development of Quality System Programs. Part IP includes requirements for materials and components, design, fabrication, assembly, erection, inspection, examination, testing, operation, and maintenance of piping, and applies to piping for liquid and gaseous hydrogen and joints connecting piping to equipment. It describes four basic methods for design of components for internal pressure and lists the dimensional standards for piping components. Part PL sets forth the requirements for materials, components, design, fabrication, assembly, erection, inspection, examination, testing, operation, and maintenance of hydrogen pipelines. History Jimmy E. Meyer revised the initial inputs of Louis E. Hayden who authored this chapter for the fourth edition. The current online edition has been reviewed by George Rawls and updated by Jimmy E. Meyer.