The Interactive Language Implementation System (ILIS) is a tool for implementing language processors. It is fast enough for conventional compilers and general enough for processing natural languages. ILIS is built around a language for writing grammars. Unlike most compiler-compilers, the language includes a full range of semantic operators that reduce or eliminate the need for invoking other programming languages during a translation. ILIS is also highly interactive: It has facilities for tracing a parse and for adding or deleting grammar rules dynamically. This paper describes the features of ILIS and its use in several different projects.