Some of the issues below are commonly recognized as unsolved problems; i.e., it is generally agreed that no solution is known. Others may be described as controversies; i.e., while there is no common agreement about the answer, there are established schools of thought that believe they have a correct answer.
Is it possible to formally delineate languages from each other? That is to say, is it possible to use linguistic (rather than social) criteria to draw a clear boundary between two closely related languages with a dialect continuum between their respective standard forms (e.g. Occitan and Catalan)?
Unclassified languages (languages whose genetic affiliation has not been established, mostly due to lack of reliable data) make up about 38 of the 6,000-7,000 languages spoken in the world. An additional 45 languages are classified as language isolates, with no demonstrable relationship to other languages.
Is the human ability to use syntax based on innate mental structures or is syntactic speech the function of intelligence and interaction with other humans? The question is closely related to those of language emergence and acquisition.
The language acquisition device: How localized is language in the brain? Is there a particular area in the brain responsible for the development of language abilities or is it only partially localized?