  Different theoretical methods used for the description of diffractive
processes in small-x deep inelastic scattering are reviewed. The semiclassical
approach, where a partonic fluctuation of the incoming virtual photon scatters
off a superposition of target colour fields, is used to explain the basic
physical effects. In this approach, diffraction occurs if the emerging partonic
state is in a colour singlet, thus fragmenting independently of the target.
Other approaches, such as the idea of the pomeron structure function and two
gluon exchange calculations, are also discussed in some detail. Particular
attention is paid to the close relation between the semiclassical approach and
the method of diffractive parton distributions, which is linked to the relation
between the target rest frame and the Breit frame point of view. While the main
focus is on diffractive structure functions, basic issues in the diffractive
production of mesons and of other less inclusive final states are also
discussed. Models of the proton colour field, which can be converted into
predictions for diffractive cross sections using the semiclassical approach,
are presented. The concluding overview of recent experimental results is very
brief and mainly serves to illustrate implications of the theoretical methods
presented.
