  Currently favoured extensions of the Standard Model typically contain `flaton
fields' defined as fields with large vacuum expectation values (vevs) and
almost flat potentials. If a flaton field is trapped at the origin in the early
universe, one expects `thermal inflation' to take place before it rolls away to
the true vacuum, because the finite-temperature correction to the potential
will hold it at the origin until the temperature falls below $1\TeV$ or so. In
the first part of the paper, that expectation is confirmed by an estimate of
the finite temperature corrections and of the tunneling rate to the true
vacuum, paying careful attention to the validity of the approximations that are
used. The second part of the paper considers topological defects which may be
produced at the end of an era of thermal inflation. If the flaton fields
associated with the era are GUT higgs fields, then its end corresponds to the
GUT phase transition. In that case monopoles (as well as GUT higgs particles)
will have to be diluted by a second era of thermal inflation. Such an era will
not affect the cosmology of GUT strings, for which the crucial parameter is the
string mass per unit length. Because of the flat Higgs potential, the GUT
symmetry breaking scale required for the strings to be a candidate for the
origin of large scale structure and the cmb anisotropy is about three times
bigger than usual, but given the uncertainties it is still compatible with the
one required by the unification of the Standard Model gauge couplings. The
cosmology of textures and of global monopoles is unaffected by the flatness of
the potential.
