The Ache were first mentioned by Jesuit historians who described them in derogatory terms as living just like animals. Undoubtedly, the Ache provided a striking contrast to the elegant and "civilized" Guaraní horticulturist peoples who inhabited the region of the Paraná and Paraguay rivers at the time of the Conquest. The Ache lived in tiny bands that subsisted entirely off wild plants and animals. They had no leaders, no permanent settlements, and very simple tools and ornaments. Lozano (1873-1874) was the first to refer to them directly by name: "Only slightly less barbaric (than the caaiguas), is the guachagui nation, although easier to tame.... They go completely naked, men and women, except that they cover their backs with a piece of woven material to guard against thorns.... And seeing or sensing strangers in their country they flee quickly without allowing one to speak with them, because they believe either that they are going to be killed, or they are being sought in order to steal their women, like they do to each other...."
The Ache were pursued relentlessly, by missionaries, enemy Indians, and slave traders until the second half of the twentieth century. For this reason, relations between them and all outsiders were overtly hostile, and very little was known about them until quite recently. In 1908 a German immigrant to Paraguay, Federico Maintzhusen, managed to make peaceful contact with a small band and published some information about them. Later, when Maintzhusen returned to Germany, this band disappeared or was assimilated into the Paraguayan population. In 1959 half of the Ypety band walked out of the forest to live with Jesús Pereira, a man who had treated one of them well when he was working as a captive slave. A short while later the other half of this band joined their kin at Pereira's farm. Pereira used this group to initiate contact with the nearby Yvytyruzu Ache between 1962 and 1963, and from these two bands came the first good ethnographic information on the Ache. More than half these Ache died from virgin-soil epidemics (epidemics that strike regions where people have no immunity to exotic epidemic diseases) within a few years of peaceful contact.
In 1968 Pereira moved his Ache reservation into the home range of the Northern Ache in order to contact and subdue them. The first band of Northern Ache was finally contacted and brought to the reservation in 1970. By 1978 all of the Northern Ache had either been convinced to join the Ache reservation, or had died from virgin-soil epidemics that swept the Northern group after first peaceful contact. About one-half of the population died from these epidemics. Finally, missionaries from the United States made peaceful contact with the Ñacunday Ache in 1976, and no more independent forest-living bands remained. The four Ache groups now live in four reservation-type settlements where they have learned agricultural practices and occasionally participate in wage labor. Many Ache also continue to return to the forest for several days or weeks at a time to hunt and gather as they did before contact.