  The QCD expectations concerning the deep inelastic lepton - hadron scattering
at low $x$ and their phenomenological implications for HERA are summarised.
Theoretical predictions for the structure function $F_2(x,Q^2)$ based on the
leading log$1/x$ resummation are presented and compared with the results
obtained from the Altarelli-Parisi equations. Theoretical predictions are
confronted with the recent data from HERA. The role of studying the final
states in deep inelastic scattering for revealing the dynamics at low $x$ is
emphasised and some dedicated measurements like deep inelastic plus jet events,
transverse energy flow and dijet production in deep inelastic scattering are
discussed.
