The determination of the calorific value of natural gases is of great economic importance in gas supply. In this paper, different methods for calorific value determination are described. Besides gas calorimetry using the classical procedure of burning a gas sample, different methods are available: analytical methods which are based on the analysis of the gas sample, correlative methods which allow the calorific value to be calculated as a function of characteristic physical measurands, stoichiometric methods which make use of the almost linear correlation between oxygen required for combustion and calorific value, and state reconstruction which is based on calorific value calculation by offline simulation of the whole gas supply network. The principles of the different methods are described in this paper.