  Reionization is a process whereby hydrogen (and helium) in the Universe is
ionized by the radiation from first luminous sources. Theoretically, the
importance of the reionization lies in its close coupling with the formation of
first cosmic structures and hence there is considerable effort in modelling the
process. We give a pedagogic overview of different analytical approaches used
for modelling reionization. We also discuss different observations related to
reionization and show how to use them for constraining the reionization
history.
