Inflation in the warm and cold regimes

It is now understood that inflation dynamics comes in two forms, isentropic or cold inflation and nonisentropic or warm inflation. In the former, inflation occurs without radiation production, whereas in the latter both radiation production and inflation occur concurrently. Recent, detailed, quantum field theory calculations have shown that many generic inflation models, including hybrid inflation, which were believed only to have cold inflation regimes, in fact have regimes of both warm and cold inflation. These results dispel many foregone assumptions generally made up to now about inflation models and bring to the fore various elementary issues that must be addressed to do reliable calculations from inflation models. Here I review these results and issues. I then show that warm inflation has intrinsic model independent features that makes it natural or equivalently have no ``eta problem''. Next density perturbations and observational consequences of warm inflation are discussed. Finally the implications of warm inflation to model building and physics beyond the Standard Model are outlined.