Let Φ be a nuclear space and let Φ ′ β denote its strong dual. In this work we establish the oneto-one correspondence between infinitely divisible measures on Φ ′ β and Lévy processes taking values in Φ ′ β . Moreover, we prove the Lévy-Itô decomposition, the Lévy-Khintchine formula and the existence of càdlàg versions for Φ ′ β -valued Lévy processes. A characterization for Lévy measures on Φ ′ β is also established. Finally, we prove the Lévy-Khintchine formula for infinitely divisible measures on Φ ′ β .