In the absence of a clear dark matter signal, models beyond WIMPs have recently gained a lot of traction. One of the leading alternatives is the Feebly Interacting Massive Particle (FIMP) paradigm, where the dark matter abundance is set by the leakage of energy from the visible sector by the so-called freeze-in mechanism. Relic density calculations in freeze-in are generally sensitive to higher temperatures, and hence to a number of in-medium effects, such as phase transitions, spin statistics of quantum gases, thermal masses and reheating temperature. 

In this talk, I will discuss these effects and present a reformulation of freeze-in which facilitates their inclusion. I will also quantify their relevance by using the example of a simple scalar singlet model.