This book constitutes selected revised papers of the 15th International Conference, NooJ 2021, held in Besancon, France, in June 2021. Due to the COVID-19 pandemic the conference was held online. NooJ is a linguistic development environment that allows linguists to formalize several levels of linguistic phenomena. NooJ provides linguists with tools to develop dictionaries, regular grammars, context-free grammars, context-sensitive grammars and unrestricted grammars as well as their graphical equivalent to formalize each linguistic phenomenon. The 20 full papers presented were carefully reviewed and selected from 62 submissions. The papers are organized in the following topics: linguistic formalization and analysis, digital humanities and teaching, natural language processing applications.