  We have performed detailed studies of non-linear structure formation in
cosmological models with light neutrinos. For the first time the effect of
neutrino thermal velocities has been included in a consistent way, and the
effect on the matter power spectrum is found to be significant. The effect is
large enough to be measured in future, high precision surveys. Additionally, we
provide a simple but accurate analytic expression for the suppression of
fluctuation power due to massive neutrinos. Finally, we describe a simple and
fast method for including the effect of massive neutrinos in large-scale N-body
simulations which is accurate at the 1% level for \sum m_nu < 0.15 eV.
