The aim of this paper is to investigate verbal stem reduplication in several Tupi-Guarani languages. Tupi-Guarani languages in general display both a monosyllabic and a disyllabic reduplication pattern. The paper presents first the semantics of the two patterns, then the phonological description of the reduplication patterns, both for Tupi-Guarani languages in general and for Emerillon in particular. On the semantic level, in most Tupi-Guarani languages, monosyllabic reduplication expresses "event-internal" plurality, while disyllabic reduplication conveys "event-external" plurality in an iconic way. In Emerillon, "event-internal" plurality may also be expressed by disyllabic reduplication. On the phonological level, while in other Tupi-Guarani languages the patterns copy the final syllables of the stem, in Emerillon the reduplication clearly affects the initial syllables. Diachronic observations explain why in some Tupi-Guarani languages, "event-internal" plurality may also be expressed by disyllabic reduplication : since monosyllabic reduplication has disappeared as a productive phenomenon from many Tupi-Guarani languages, disyllabic reduplication took on its meaning of "event-internal" plurality.