They belong to the « centum » branch of the Indo-European languages. With Hittite, they demonstrate the most anciently recorded Indo-European linguistic material but show an advanced formal development comparable with Greek and Sanskrit.

Although their vocabulary is heavily influenced by the native (non Indo-European ?) environment they retain Indo-European grammatical structures. Lydian is known from about 50 inscriptions mainly from Sardis. Lycian is known form some 200 inscriptions and coin legends, including some Lycian - Greek bilinguals.