The most complex cultures emerged, how ever, in the Andean northwest , where sedentary agricultural practices developed from about 500 BC. Irrigation permitted the intensive cultivation of staple crops like maize, quinoa, squash and potatoes and this, combined with the domestication of animals like the llama, facilitated the growth of rich material cultures, as attested to by the archeological record. The most important early sedentary culture is the Tafi one of the Tucuman region, whose people sculpted intriguing stone menhirs incised with geometric designs, feline shapes and human faces. This initial period saw the later development of Catamarca's Condorhuasi culture, renowned for its distinctive and beautifully patterned ceramics. From about 600 AD, metallurgical technologies developed, which saw the use of bronze for items as elaborate as ceremonial axes and chest-plates, as best witnessed in the Aguada civilization, whose territory also centred on Catamarca. From about 850 AD, the increasing organization of Andean groups is demonstrated by the appearance of fortified urban settlements, which, though relatively humble by the standards of the great civilizations further north, were nevertheless built in stone and had populations of up to a few thousand. Three important Diaguita cultures emerge: Sanagasta; Belen; and Santa Maria , whose overlapping zones of influence stretched from Salta through to San Juan, and which are notable for their elaborately painted ceramics, anthropomorphic funeral urns, superb metalwork, and the use of agricultural terracing. Further north, separate cultures develop in the Humahuaca region of Jujuy, including those of Tilcara and El Alfarcito, both of which have evidence of a marked use of hallucinogenic substances.

These Andean cultures engaged in trade with their counterparts on the Pacific side of the Andes and north into what is now Bolivia. Trade networks were vastly increased once the area came under the sway of pan-Andean empires: first that of Bolivia's great city, Tiahuanaco , which probably influenced Condorhuasi culture; and, from 1480, that of the Incas , who incorporated the area into Kollasuyo, their southernmost administrative region. Incredibly well-preserved finds, such as recent excavation of three ritually sacrificed mummies at the summit of 6739-metre Cerro Llulliallaco - the world's highest archeological discovery - are helping to reveal the extent of this influence in terms of customs, religion and dress.

In the early sixteenth century, before the arrival of Europeans, Argentina's indigenous population was probably in the region of 400,000, an estimated two-thirds of whom lived in the northwest - Andean groups such as the Diaguitas, the Omaguacas of Jujuy's Humahuaca Valley, the Atacamenos of the far northwestern puna, and the Tonocotes of Santiago del Estero. Other relatively densely settled areas included the central sierras of Cordoba and San Luis, where the Comechingones and the Sanavirones lived. The Cuyo region was home to semi-sedentary Huarpes; while to the south and east of them lived various Tehuelche tribes, often referred to generically by the Spanish as Pampas Indians or, further south, Patagones. Tierra del Fuego was inhabited by Selk'nam and Mannekenk, as well as the Yamana sea-goers. The Chaco region was home to a bewildering variety of shifting nomadic groups, including Chiriguanos, the Lule-Vilela, Wichi, and groups of the Guaycuru nation, like the Abipone and Qom. The northeastern areas of El Litoral and Mesopotamia were inhabited by the Kaingang, the Charrua and Guarani groups.

The first group to encounter the Spanish were probably the nomadic Querandi of the Pampas region - the northernmost group of the wider Tehuelche culture. They lived in temporary shelters and hunted guanaco and rhea with bolas ( boleadores ): weighted thongs used to bring down their prey. Though they put up determined resistance to the Spanish for several decades, their culture was eliminated during the subsequent colonial period - a fate that was to be shared by many others.