Louis Trolle Hjelmslev (Danish: [ˈjɛlˀmslew]; October 3, 1899, Copenhagen – May 30, 1965, Copenhagen) was a Danish linguist whose ideas formed the basis of the Copenhagen School of linguistics. Born into an academic family (his father was the mathematician Johannes Hjelmslev), Hjelmslev studied comparative linguistics in Copenhagen, Prague and Paris (with a.o. Antoine Meillet and Joseph Vendryes). In 1931, he founded the Cercle Linguistique de Copenhague. Together with Hans Jørgen Uldall he developed a structural theory of language which he called glossematics, which developed the semiotic theory of Ferdinand de Saussure. Glossematics as a theory of language is characterized by a high degree of formalism, it is interested only in describing the formal characteristics of language, and has a high degree of logical rigour. The theory never became widely influential, but has been adopted by post-structuralist philosophers as a possible alternative to the dominant Saussurean linguistic paradigm.
The Linguistic Circle of Copenhagen was founded by Hjelmslev and a group of Danish colleagues on 24 September 1931. Their main inspiration was the Prague Linguistic Circle, which had been founded in 1926. It was, in the first place, a forum for discussion of theoretical and methodological problems in linguistics. Initially, their interest lay mainly in developing an alternative concept of the phoneme, but it later developed into a complete theory which was coined glossematics, and was notably influenced by structuralism. Membership of the group grew rapidly and a significant list of publications resulted, including an irregular series of larger works under the name Travaux du Cercle Linguistique de Copenhague. A Bulletin was produced, followed by an international journal for structuralistic research in language, Acta Linguistica (later called Acta Linguistica Hafniensia), which was founded with the members of the Prague Linguistic Circle. It was, at that time, the sole journal explicitly dedicated to structuralism. With one short break from 1934 to 1937, while he lectured at the university of Aarhus, Hjelmslev acted as chairman of the Circle until shortly before his death in 1965.