I will summarize the history of IGM temperature measurements from the Lyman-alpha forest as well as the theory for the IGM temperature after reionization. I will show that the simplest theory for thermal evolution, which has little parametric freedom, works remarkably well at reproducing recent measurements spanning 1.5 < z <4.5. This agreement allows little room for extra sources of heat beyond photoionization. At z > 5, large fluctuations in the opacity of the Lyman-alpha forest are observed on 100 Mpc scales (with some regions being completely opaque — these are the famous Gunn-Peterson troughs). I will show that these fluctuations at z~5.5 either indicate large spatial fluctuations in the temperature or in the ionizing background. For both possibilities, the amplitude of these fluctuations yield insights into the process of cosmological reionization: that it was quite extended (it temperature is the culprit) and that it ended at z~6 (for both). Finally, at low redshifts it becomes difficult to see cosmic gas with the Lyman-alpha forest and as a result we do not have a reliable census for where it lies. I will talk about the possibility of using dispersion measures to FRBs (and briefly why one cannot do this with non-variable sources) to map out the distribution of cosmic gas.