Radioactivities from Stellar Nucleosynthesis: Slow and Fast Clocks in the Solar System

I briefly review the relevance of radioactive nuclei for timing events of interest in the evolution of the Solar System. The related techniques are based on nuclear radio-chronometry of terrestrial or meteoritic samples. A special attention is paid to short lived ($\bar \tau \le$ 100 Myr) isotopes produced by stellar nucleosynthesis: I mention the indirect information we have on their presence in the early solar nebula, and discuss their use both for establishing very precise isochrons in the initial evolutionary stages of the Solar System, and for inferring the history of nucleosynthesis prior to the Sun's formation. In particular, from the existing record of a few important nuclei (26Al, 60Fe, 205Pb, 41Ca among others) it appears that the protosolar nebula was subject to late contaminations both by spallation reactions in the fast winds of the early Sun, and by a nearby star. The recent suggestions that this last was of low or intermediate mass are also discussed