An ANTLR (Another Tool for Language Recognition) developer designs and builds software applications using ANTLR, a powerful parser generator. They define grammars for specific programming languages or data formats, and use ANTLR to generate parsers that can process files or streams in those languages or formats. They also debug, maintain and improve these parsers. An ANTLR developer might work on compilers, interpreters, or tools for data extraction and transformation. They need to understand language theory, including concepts like syntax trees and grammars, and be proficient in Java, as ANTLR itself is written in it.