In this thesis we investigate nonlocal fields in physics at finite temperature and density. We first investigate the thermodynamic properties of a nonlocal tachyon motivated by the nonlocal structure in string field theory. We use previously developed perturbative methods for nonlocal fields to calculate the partition function and the equation of state in the high temperature limit. We find that in these models the tachyons undergo a second order phase transition. We compare our results with those of ordinary scalar field theory. We also calculate the one loop finite temperature effective potential. We then investigate a nolocally modified effective field theory for nuclear matter. We pay particular attention to the effect of the modification on the two-loop diagrams. We then compare to the conventional case. We find that while we do end up with a softer behavior in the loop contributions this leads to only a minor reduction in the magnitude of the coupling constants.