Languages can vary considerably according to several factors, namely geographical, social or pragmatic. This paper shall focus on diatopic or geolinguistic variation of European Portuguese, the overall designation that encompasses the dialects spoken both in mainland Portugal and in the islands of Azores and Madeira, as well as by the immigrants scattered through Europe. Starting from the discussion of the dichotomy language versus dialect, we will present the various linguistic atlases designed by Leite de Vasconcelos, Paiva Boléo and Lindley Cintra. Based on Lindley Cintra’s new proposal from 1971, the main isoglosses in Portugal shall be presented and explained in order to allow the distinction of the Galician, the northern Portuguese and the mid-southern Portuguese dialects, the latter being where the islands are included. A final word shall be directed to the Mirandese, the second official language in Portugal.