This chapter presents various specifications for the materials used in the construction of boilers. The first section of this chapter traces the chronological evolution of materials and associated technologies, from the need for materials to accommodate riveted construction to the acceptance of fusion welding as a fabrication process. Included in this discussion are the application of advanced materials, the revisions to the basis for setting allowable stress values, and the acceptance of Material Specifications other than those approved by ASTM. The next two sections discuss Code Section II, Part A–Ferrous Material Specifications and Part B–Nonferrous Material Specifications. The discussion covers the following: Guideline on the approval of new materials under the ASME Boiler and Pressure Vessel (BPV) Code; Submittal of Technical Inquiries to the Boiler and Pressure Vessel Committee; and Guidelines on Multiple Marking of Materials. The fourth section provides the basis for the Specifications and Standards covered by Section II, Part C–Specification for Welding Rods, Electrodes, and Filler Metals, and their relations to the ANSI/AWS specifications. Included in the discussion are the electrode classification system, material descriptions, welding material applications, welding material procurement, and filler metal certification. The fifth section reviews important aspects of Section II, Part D–Properties. The discussion covers the properties of ferrous and nonferrous alloys used in the design of components for the BPV and Nuclear Construction Codes. The final section discusses Section II, Part A and Part B—International Material Specifications. ASME Section II includes rules for acceptance of material specifications of recognized National or International organizations other than ASTM. The section also discusses the process of adoption of the Committee for Standardization specifications in Europe with consequences on the corresponding ASME specifications.