Attempto Controlled English (ACE) is a controlled natural language, i.e. a precisely defined subset of English that can automatically and unambiguously be translated into first-order logic. ACE may seem to be completely natural, but is actually a formal language, concretely it is a first-order logic language with an English syntax. Thus ACE is human and machine understandable. ACE was originally intended to specify software, but has since been used as a general knowledge representation language in several application domains, most recently for the semantic web. ACE is supported by a number of tools, predominantly by the Attempto Parsing Engine (APE) that translates ACE texts into Discourse Representation Structures (DRS), a variant of first-order logic. Other tools include the Attempto Reasoner RACE, the AceRules system, the ACE View plug-in for the Protégé ontology editor, AceWiki, and the OWL verbaliser.