This document serves as a reference manual for the High Level Assembler (HLASM) for z/OS, z/VM, and z/VSE. It covers topics such as program structures, machine instruction statements, assembler instruction statements, macro language, and conditional assembly language. The manual provides information on addressing, constants, system variable symbols, and special characters. It also includes chapters on writing macro instructions and conditional assembly instructions, as well as appendices, notices, a bibliography, and an index.