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 } \lesssim 0.15 { eV } .