Let be a discrete valuation ring of unequal characteristic and let
be its fraction field. The aim of this talk is to give the
classification of models of
, i.e.
finite and flat group schemes over which are isomorphic to
over . The main features of this
classification are the following: 1) the parameters can be easily
interpretated; 2) the description of the models is explict, i.e. it is
given in terms of equations; 3) any model can be seen as the kernel
of an exact sequence which coincides generically with the Kummer
sequence. This sequence let us to generalize the Kummer Theory to
describe torsors under these group schemes. The main tool which we use
is the Sekiguchi-Suwa Theory, which we will briefly recall. If we
will have enough time we will compare our work with the recent works
of Breuil and Kisin about the classification of finite and flat goup
schemes over a d.v.r.