The aim of this work is to describe in detail the POWHEG method, first
suggested by one of the authors, for interfacing parton-shower generators with
NLO QCD computations. We describe the method in its full generality, and then
specify its features in two subtraction frameworks for NLO calculations: the
Catani-Seymour and the Frixione-Kunszt-Signer approach. Two examples are
discussed in detail in both approaches: the production of hadrons in e+e-
collisions, and the Drell-Yan vector-boson production in hadronic collisions.