The valley is parallel to the alpine ridge being set on the insubric line, a system of faults that mark the welding between the ancient Europe and the Adriatic plate detached from Gondwana. It therefore separates the Central-Eastern Alps (western Rhaetian Alps) from the South-Eastern Alps (Alps and Prealps of Bergamo and the Orobian Alps). It is 120 km long and 66 km wide.
The river Adda, descending from the Cancano valley, until it merges into Lake Como, traces the course of the Valtellina, which touches to the east the Trentino-Alto Adige, to the south the provinces of Bergamo and Brescia, to the north the Swiss lands of Grisons. With the exception of the Val Poschiavo (the Italian-speaking Swiss valley leading to St. Moritz via the Bernina pass), from all these areas the Valtellina is separated by mountains more or less high, some very high and among the first in Europe ; it is sufficient to name the Bernina massif, the Ortles, the Cevedale the Gran Zebrù and the Adamello-Presanella group. To the west the Valtellina begins with the Pian di Spagna, a vast plateau once marshy, corresponding to the insertion of the Adda in the Lario, now largely reclaimed, and borders the provinces of Como and Lecco. The valley then develops into a series of basins, closed by bottlenecks when the two mountain ridges approach each other. Finally, the mountains almost close the valley, leaving only a small and difficult access to a last, vast amphitheater, which forms the basin (formerly the countryside) of Bormio. They depart from the Bormina basin: to the west the short valley of Pedenosso, where the Viola valley and the Dentro valley converge: the latter leads, through the Foscagno pass, to the Livigno basin and then to the Engadine and the Grisons; to the north the valley of the Braulio, along which the road of the Stelvio pass rises; to the east, the Valfurva, with the valleys of the Zebrù valley, the valley of the Forni and the Gavia valley which, through the namesake and rough pass, connects Valtellina and Camonica valley.
The main passes of the Valtellina are the Stelvio (which with its 2,758 meters is the highest in Italy and the second in Europe),  often protagonist of the Giro d’Italia, which leads to the Venosta Valley (Alto Adige), the Gavia pass (2,621 m) towards the Alta Val Camonica, the San Marco pass (1,992 m) towards the Brembana valley and the Aprica pass (1,200 m) towards the Camonica di Edolo valley, the Mortirolo pass (1,852 m) ) towards Val Camonica.
The main city and provincial capital is Sondrio; other important places are Tirano, Morbegno, Teglio, Sondalo, Bormio and Livigno. These last two are very popular tourist destinations for their ski resorts. Livigno also has the advantage of being a free port while Bormio has the advantage of having excellent thermal waters known since antiquity (Terme di Plinio). The basin of Livigno, as regards the conformation of the water basins, is instead located in the basin of the Inn
The Trophy of the Alps recalls the alpine tribe of the Vennoneti, probably the ancient inhabitants of the Valtellina.
The valley was colonized, since ancient times, by populations of Celtic, Ligurian and Etruscan origins. In particular Virgil, Pliny the Younger (from Como) and Martial tell of how, in the pre-Roman age, the first ligust and Etruscan settlements had imported the vine from the Cinque Terre and Lunigiana areas to Valtellina.
The ancient Ligurian people settled precisely, as well as on a long coast that went from Marseille to Luni, along the northern Apennine ridge, on both sides of the Western Alps. Grouped in lineages or tribes, in particular the Ligurian Staziellis, they acquired – since they already knew the vine – from the Greeks the first rudiments of winemaking.
It may therefore have been the Ligurian Gauls who introduced the “Nebbiolo” grape variety in Valtellina, when they colonized it. Their passage through the Val Chiavenna gave the appellative “chiavennasco” to the original vine. But this is only a mere hypothesis.
The High Middle Ages
The Valtellina after having belonged to the Roman Empire ended in 568 AD under the Lombard rule. Numerous Arimannic groups settled in these lands, among which the powerful Crotti Arimanni Longobardi, that is Longobard warriors on horseback, who settled in this territory in the High Middle Ages from Bergamo, contributing with their own name to the toponymy of various areas of Valtellina. Subsequently it was the turn of the dominion of the people of the Franks, to then pass under the bishops princes.
The Middle Ages
During the Late Middle Ages Valtellina followed the fate of the remaining Lombardy. From the ecclesiastical point of view it was always subject to the bishops of Como, while civilly after being subjected to the Municipality of Como and the bishop of Como it was incorporated in the mid-fourteenth century in the Duchy of Milan. The inhabitants of the neighboring Grisons, who had already entered the Valtellina several times, in 1512, app