In the early universe, the Standard Model particles formed a hot thermal bath. We highlight the importance of finite temperature corrections in these conditions on various production mechanisms of Dark Matter, primarily through temperature dependent masses and scalar vevs. We first consider a freeze-in model where kinematic thresholds determine the relic abundance. We then consider a model where the freeze-in rate is dramatically increased when a new scalar obtains a vev. Finally, we present a qualitatively new production mechanism for Dark Matter, where Dark Matter decay is allowed for a limited amount of time just before the electroweak phase transition.