LANG-PAK is a monograph concerning an interactive language design system which purports to aid a language designer in designing simple interactive computing languages. Suggested uses of LANG-PAK include specialized interactive languages for information retrieval systems, a graphics display language, an input-output data definition language, a language for designing forms, and a language to interface to a statistics and forecasting system. LANG-PAK is a translator writing system which gives a language designer the facility to specify the syntax and semantics of a language and the facility to parse trial statements in the defined language. Emphasis should be placed on the idea of simple languages which can be designed using LANG-PAK. LANG-PAK would have difficulty coping with complex languages due to the authors' choice of a requirements for the syntax rules of the language a designer is working with.