The aim of this thesis is to analyse the language features in M.B. Landstad's edition of Norwegian medieval ballads that are different from the present-day standard Norwegian, and to determine which of these features correspond to the living Telemark dialect of Landstad's time, and which are vestiges of Old Norse and Old Norwegian. The hypothesis is that the folk poetry that Landstad collected in the Telemark district bears not only features of the local dialect, but also has, due to its bound form (rhythm, rhyme), kept some of the language features from the time of its origin (14th/15th century) up to the time of collection (the middle of 19th century). The thesis concentrates on the analyse of various language features in the field of morphology, sound changes and vocabulary. The analysis and classification of the language features will proceed on the basis of literature about the historical development of Norwegian dialects and by comparison with other texts written in the Telemark dialect and texts written in Old Norse and Old Norwegian