Adjectives carry a lot of discourse semantics, often being the core elements to understand the literal, emotional, cultural, and metaphoric meaning of a sentence. Their formal semantics has been investigated specially in lexical semantics and formal linguistics, with some contributions from formal ontology and the semantic web. However, no standard formal treatment of adjectives is available yet, which is capable to address the concerns of both theoretical and computational natural language understanding. In this paper we summarize the existing approaches, present some alternative solutions to approximate a rigorous but pragmatic representation, and describe an implementation of a lightweight adjective ontology as a core resource in FRED, a state-of-the-art open knowledge extraction tool.