The Department of Theoretical Linguistics takes interest in linguistic theories which seek to answer the question what a language is and why languages have the properties they have. The theoretical apparatus available in different linguistic theories is tested against a broad range of data, taken from a number of languages, in particular English and Polish. The theoretical model whose validity is primarily tested is the Minimalist Program of Chomsky (1995, 2000, et seq.), which is widely adopted in linguistic research all over the world. The data analysed are to shed light on the universal character of the mechanisms proposed in the Minimalist Program, and if need be, lead to the modification of the theory. The research carried out is based on corpus analysis and statistical data. The study of linguistic theories is also meant to have practical applications, one of which is to help in corpus annotation. Another practical  application is to guide the development of e-learning platforms.