Heavy quark physics requires a precise experimental determination of the fraction of produced events containing heavy quarks. A number of methods has been established and those most widely used at the HERA experiments are discussed below. The identification algorithms are generally similar among the different high energy physics experiments in that characteristic signatures of the decay processes of heavy hadrons are exploited. Experimentally, the different techniques are somewhat complementary and range from the full reconstruction of the invariant mass using all decay particles to the measurement of the lifetime distribution of the hadrons by means of displaced vertices or track parameters. The methods, which are described in detail in the following, are often combined to enhance the significance of the heavy quark signal separation from the light-quark background.
