There is a growing demand on remote temperature sensing with spatial resolution at the micrometer scale and below. Ratiometric luminescence thermometry is a promising and non-invasive methodology for those length scales. Potential application areas range from in situ monitoring of temperature changes in chemical reactions, over in vivo bioimaging to investigation of fundamental thermodynamic phenomena at the nanoscale. A particularly simple way of luminescence thermometry employs an ensemble of non-interacting luminescent centers with two thermally coupled and radiatively emitting states from the same electron configuration. The luminescence intensity ratio then follows Boltzmann’s law. Trivalent lanthanoids with their narrow line 4fn-4fn luminescence doped into crystalline powders have emerged for this type of thermometry. The ultimate desire to design such thermometers for the application of interest requires, however, a careful understanding of both thermodynamic and kinetic concepts of their performance. In this presentation, I will give a general overview over the foundations of this so-called Boltzmann-type thermometers. It will be demonstrated that every luminescent Boltzmann thermometer can be used for precise temperature measurements in only a small temperature window and what are strategies to overcome this obstacle. I will also show the relevance of excited state kinetics and how decisive the choice of the surrounding host compound for the lanthanoid ions is to control Boltzmann-type behavior of a luminescent thermometer. The interplay with energy transfer pathways will also be shortly highlighted. The presentation will conclude with a perspective on how seeking a better understanding of luminescent thermometers can also help understand the efficiency of phosphors quite generally.