  We review recent developments in the theoretical description of inclusive
single-hadron production at next-to-leading order in the parton model of
quantum chromodynamics. Fragmentation functions are extracted from fits to data
of inclusive pion and kaon production in e^+e^- annihilation at different
centre-of-mass energies. Exploiting sensitivity to the scaling violation, one
can simultaneously fit the asymptotic scale parameter Lambda so as to obtain an
independent determination of the strong coupling constant alpha_s. Owing to the
factorization theorem, the fragmentation functions only depend on the species
of the produced particles, but not on the process by which they are produced.
This allows one to make absolute theoretical predictions for inclusive pion and
kaon production in other types of experiments such as photon-photon,
photon-hadron, or hadron-hadron scattering. Recent data of photoproduction
taken by the H1 and ZEUS Collaborations at DESY HERA nicely agree with such
next-to-leading-order predictions.
