Flavour tagging of parton jets

The jets of hadrons detected in high energy reactions come usually from a variety of partons produced by many contributing hard subprocesses. Since partons have not yet been observed directly, tagging of parton jets is so far the only way to investigate interactions and fragmentation of a given type of partons. Tagging methods and some selected results are presented for lepton induced and hadronic collisions. The relevance of these techniques is discussed to detailed studies of QCD subprocesses, fragmentation schemes for quarks, gluons and diquarks, interactions of partons with nuclear matter and electroweak asymmetries. Based on recent data it is finally argued that diffractive systems are jets of tagged parton systems. 53 refs