  A range of experimental results point to the existence of a massive neutrino.
The recent high precision measurements of the cosmic microwave background and
the large scale surveys of galaxies can be used to place an upper bound on this
mass. In this paper we perform a thorough analysis of all assumptions that go
into obtaining a credible limit on $\sum m_{\nu}$. In particular we explore the
impact of extending parameter space beyond the current standard cosmological
model, the importance of priors and the uncertainties due to biasing in large
scale structure. We find that the mass constraints are independent of the
choice of parameterization as well as the inclusion of spatial curvature. The
results of including the possibility of dark energy and tensors perturbations
are shown to depend critically on the data sets used. The difference between an
upper bound of 2.2 eV, assuming generic initial conditions, and an upper bound
of 0.63 eV, assuming adiabaticity and a galaxy bias of 1, demonstrate the
dependence of such a constraint on the assumptions in the analysis.
