Abstract : This document is a users' guide to LapItUp, the program which is used to add words to the Master Lexicon component of the JANUS natural language system, where syntactic, morphological, orthographic and semantic information particular to individual words and phrases is stored. The Master Lexicon is designed to serve all the lexical needs of the two grammars which form part of the JANUS system: RUS, an ATN parsing grammar, and Nigel, a systemic generation grammar. The user's guide contains a description of the organization of the Master Lexicon, instructions for adding words to the lexicon, and descriptions of each of the features that can be acquired using LapItUp. There are two important aspects to the Master Lexicon: the wordclass organization, which organizes the syntactic and morphological features used in lexical item specification, and the lexical item organization. In addition to a feature specification, the latter contains cross-indexing information or properties, semantic pointers, and record-keeping information. LapItup uses the wordclass hierarchy to guide the use through a series of choices, presented in the form of menus of features with accompanying descriptive text. In addition, LapItUp contains facilities for acquiring morphological information, properties, and the other kinds of auxiliary information. Keywords: Artificial intelligence.