  The spectroscopy of free radicals and radical containing entrance-channel
complexes embedded in superfluid helium nano-droplets is reviewed. The
collection of dopants inside individual droplets in the beam represents a
micro-canonical ensemble, and as such each droplet may be considered an
isolated cryo-reactor. The unique properties of the droplets, namely their low
temperature (0.4 K) and fast cooling rates ($\sim10^{16}$ K s$^{-1}$) provides
novel opportunities for the formation and high-resolution studies of molecular
complexes containing one or more free radicals. The production methods of
radicals are discussed in light of their applicability for embedding the
radicals in helium droplets. The spectroscopic studies performed to date on
molecular radicals and on entrance / exit-channel complexes of radicals with
stable molecules are detailed. The observed complexes provide new information
on the potential energy surfaces of several fundamental chemical reactions and
on the intermolecular interactions present in open-shell systems. Prospects of
further experiments of radicals embedded in helium droplets are discussed,
especially the possibilities to prepare and study high-energy structures and
their controlled manipulation, as well as the possibility of fundamental
physics experiments.
