  The effects induced by the quark masses in three-jet observables have been
studied in this thesis. In particular, we have explored a new method: the study
of three-jet observables at LEP, different from QCD Sum Rules and Lattice
calculations, for determining the b-quark mass. Among other advantages, it
would allow to extract the b quark mass far from threshold, in contrast to the
other methods described above and it would provide a test of the QCD
Renormalization Group predictions. From our NLO calculation, we have estimated,
by exploring the $\mu$-dependence of our prediction at m_Z, a theoretical
b-quark mass uncertainty of at most 250(MeV) in the DURHAM algorithm, 300(MeV)
for JADE and EM and 500(MeV) for the E algorithm. Furthermore, quark mass
effects are expected to be quite more important in top production at NLC (Next
Linear Collider) and our calculation would be of a great interest.
