We study models in which the universe exits reheating at temperatures in the MeV regime .
By combining light element abundance measurements with cosmic microwave background and large scale structure data we find a fairly robust lower limit on the reheating temperature of T _ { RH } \gtrsim 4 MeV at 95 % C.L .
However , if the heavy particle whose decay reheats the universe has a direct decay mode to neutrinos , there are some small islands left in parameter space where a reheating temperature as low as 1 MeV is allowed .
The derived lower bound on the reheating temperature also leads to very stringent bounds on models with n 
large extra dimensions .
For n = 2 the bound on the compactification scale is M \gtrsim 2000 TeV , and for n = 3 it is 100 TeV .
These are currently the strongest available bounds on such models .