The main objective of this lecture is to make the end users aware of the various physical phenomena and especially of the errors frequently met during temperature and heat flow measurement by infrared thermography. For that purpose, this chapter will present the three aspects of a quantitative infrared measurement that are spatial, temporal and thermal resolution. First, the spatial resolution will be discussed, showing that an increase of the matrix size does not necessarily induce an improvement of the spatial resolution. Then, a paragraph is especially dedicated to the temporal aspects, as far as many applications require at least stable frequency to high speed imaging. Last but not least, the calibration of the systems is discussed, showing that accurate measurements often need a specific home-made thermal calibration.