All software evolves, and programming languages and programming language tools are no exception. And just like in ordinary software construction, modular implementations can help ease the process of changing a language implementation and its dependent tools. However, the syntactic and semantic dependencies between language features make this a challenging problem. In this paper we detail how programming languages can be implemented in a modular fashion using the Rascal meta-programming language. Rascal supports extensible definition of concrete syntax, abstract syntax and operations on concrete and abstract syntax trees like matching, traversal and transformation. As a result, new language features can be added without having to change existing code. As a case study, we detail our solution of the LDTA'11 Tool Challenge: a modular implementation of Oberon-0, a relatively simple imperative programming language. The approach we sketch can be applied equally well to the implementation of domain-specific languages.