The Aché (pronounce Ah·CHAY) Indians are a traditional hunter-gatherer tribe living in Eastern Paraguay. They are called “Guayakí” by Guaraní speaking neighbors and in early anthropological accounts. The earliest published reports (Lozano 1873-74 summary of Jesuit accounts in the 1600s) about the Aché refer to them as “Guajagui”, a term based on the Guaraní root “Guaja” (= enemy tribe, or brother-in-law) and “gui” a common Aché suffix (meaning “essence of” or “having the property of”). The Aché language provides clues to their origin. Current analysis suggests that it is comprised of a Tupí-Guaraní lexicon, overlaid on a unique grammar structure not found in sister Guaraní languages (Eva Maria Roessler, personal communication). Genetic analyses suggest that the Aché are a group of mixed biological origin containing about 60-65% Tupí-Guaraní genes and 35-40% of their genes with affinities to the Jé language family. The Aché are also culturally and biologically distinct from the neighboring Guarani. Early descriptions of the Aché emphasized their white skin, light eye and hair color, beards, Asiatic features, and practice of cannibalism as identifying characteristics. Their subsistence practices and technology were considered extremely simple, and nomadism made them secretive and evasive. From the earliest Jesuit accounts of the Aché in the 1600s until their peaceful outside contacts in the 20th century the Aché were described as nomadic hunter-gatherers living in small bands and depending entirely on wild forest resources for subsistence. In the 20th century four different ethnolinguistic populations of Aché were contacted and pacified. They are the Northern Aché, the Yvytyruzu Aché, the Ypety Aché, and the Ñacunday Aché. Each of these populations was an endogamous dialectal group, consisting of multiple residential bands and no peaceful interaction between them.