The difference between heat losses in percentage and in absolute measures is elucidated. Good conditions for making experimental heat loss determination are outlined, i.e., to avoid varying temperatures in the ground surrounding the district heating (DH) pipes and in the pipe fluid. An overview of in-situ methods is given, and methods working on the DH pipe casing are described. Recent developments regarding quantitative thermography are also described. A comparison of the methods on an operating preinsulated transmission line is presented.