Understanding the detailed quantum dynamics of spin states for electrons confined to semiconductor nanostructures is an essential prerequisite for applications in spintronics and quantum information processing.  Although a great deal of work has been done in the past 50 years to understand spin decoherence in random ensembles or for phenomenological models, surprisingly little was known until very recently about the strongly-coupled many-body dynamics of single isolated electron spins in a realistic environment.  In any material with confined electrons and nonzero-spin nuclei, the major source of electron-spin dynamics is typically provided through the hyperfine interaction between a confined electron spin and a nuclear-spin bath.  In this talk, I will present very new results for the coupled many-body dynamics of a single electron spin in a bath of nuclear spins.  Whereas previous studies have relied on a variety of different theoretical approches to find the relevant dynamics at various different time scales and with an ambiguous range of validity, I will present a single unified and systematic approach, which recovers the correct dynamics at all times.  Moreover, this theory provides two remarkable and experimentally verifiable new predictions, which cannot be recovered using earlier approaches: (1) a non-monotonic dependence of the  decoherence rate on magnetic field, and (2) novel modulations of the decay envelope, even in the presence of a purely isotropic hyperfine coupling and for coupling to a single nuclear-spin species.