Jet Structure Studies at Lep and Hera

A summary of some recent studies in jet physics is given. Topics include leading particle production in light flavor events in e + e − annihilations, an analytical treatment of gluon and quark jets at the next-to-next-to-next-to-leading order (3NLO), and various studies performed at LEP and HERA involving separated gluon and quark jets. 1 Leading particle production in separated light quark events Separated charm (c) and bottom (b) quark events have been well studied in e + e − annihilations. c and b quarks are mostly produced at the electro-weak vertex in e + e − → qq → hadrons events, in which the primary quark q is c or b, and are relatively easy to identify. In contrast, separated up (u), down (d) and strange (s) events have not been much studied. u, d and s quarks are copiously produced during jet development, making events in which they are produced as primary quarks more difficult to identify.