abstract: In 167576 G.W.Leibniz wrote the longest mathematical treatise he ever elaborated: 'The arithmetical quadrature of the circle, ellipse and hyperbola'. In 1993 the Latin text was published for the first time by E. Knobloch. In 2004 Marc Parmentier and E. Knobloch published an bilingual Latin-French edition. The treatise gives a survey of infinitesimal geometry of that time beginning with a rigorous foundation of integration theory. Leibniz uses infinitely small and infinite quantities in a well-defined meaning and explains how to deal with these quantities. The lecture will explain the fundamental theorem regarding integration theory and will deal with some crucial issues and theorems of the treatise.