According to Grothendieck, the notion of topos is 'the bed or deep river where come to be married geometry and algebra, topology and arithmetic, mathematical logic and category theory, the world of the continuous and that of discontinuous or discrete structures'. It is what he had 'conceived of most broad to perceive with finesse, by the same language rich of geometric resonances, an 'essence' which is common to situations most distant from each other, coming from one region or another of the vast universe of mathematical things'. The aim of this work is to present a theory and a number of techniques which allow to give substance to Grothendieck's vision by building on the notion of classifying topos educed by categorical logicians.