This chapter highlights the two characteristic traits of Jesuit science in Paraguay and Rio de la Plata: that science was a practical enterprise subordinate to missionary activity, which favoured the cultivation of applied aspects of knowledge; and as a result of the inculturated style of Jesuit evangelization, segments of native lore on nature were incorporated into the discourse of European science. Jesuits observed phenomena like comets and eclipses in the Southern Cone of South America from the seventeenth century onwards. Notwithstanding a few exceptions, it seems possible to conclude that, as a whole, their scientific activities were clearly tailored for the ultimate apostolic aims of the Society: natural history was conceived as a description of the theatre where the drama of the conversion to Christianity of the native people was enacted. Astronomy was a means of setting the geographical coordinates of the mission towns; and medical botany was concerned with the health of the missionaries and their catechumens.