  This thesis consists of two parts. In the first part heavy quark production
in neutral current and charged current DIS is studied within the variable
flavor number scheme of Aivazis, Collins, Olness, and Tung (ACOT). For this
purpose all the relevant partonic subprocesses have been calculated to order
O(\alpha_s^1) for general masses and couplings taking into account massive
initial state quark-partons as needed in the variable flavor number scheme of
ACOT. By the calculation of the before missing radiative corrections to
scattering amplitudes on massive quark partons the ACOT scheme could be
completed to full order O(\alpha_s^1). These results might also prove useful
for studying the intrinsic charm content of nucleons. In the second part the
parton content of pions and real and virtual photons is analyzed in leading
order (LO) and next-to-leading oder (NLO) QCD within the framework of the
radiative parton model of Gl\"uck, Reya, and Vogt (GRV). Furthermore, the
factorization of the cross section for the process e^+ e^- -> e^+ e^- X into
fluxes of transverse/longitudinal target photons and the cross sections for
deep inelastic scattering off these target photons is investigated in detail.
It is demonstrated that the factorization remains valid also for virtual P^2 \=
0 target photons in the Bjorken limit P^2 << Q^2. However, the neglected terms
are of the order O(\sqrt{P^2/Q^2}) only and not of the order O(P^2/Q^2) as
might have been naively expected. Finally, the photon structure functions have
been calculated in lowest order perturbation theory according to the doubly
virtual box \gamma^*(P^2) \gamma^*(Q^2) -> q \bar{q}. A useful compilation of
various limits of these general expressions is provided.
