  This chapter presents a (partial) review of the information we can derive on
the early history of the Solar System from radioactive nuclei of very different
half-life, which were recognized to have been present alive in pristine solids.
In fact, radioactivities open for us a unique window on the evolution of the
solar nebula and provide tools for understanding the crucial events that
determined and accompanied the formation of the Sun. Discussing these topics
will require consideration of (at least) the following issues. i) The
determination of an age for solar system bodies, as it emerged especially from
the application of radioactive dating. ii) A synthetic account of the
measurements that proved the presence of radioactive nuclei (especially those
of half-life lower than about 100 Myr) in the Early Solar System (hereafter
ESS). iii) An explanation of their existence in terms of galactic
nucleosynthesis, and/or of local processes (either exotic or in-situ) preceding
and accompanying the formation of the Sun. This will also need some reference
to the present scenarios for star formation, as applied to the ESS.
