  The exact solution of the two-dimensional Ising model by Onsager in 1944
represents one of the landmarks in theoretical physics. On the occassion of the
fifty years of the exact solution, we give a historical review of this model.
After briefly discussing the exact solution by Onsager, we point out some of
the recent developments in this field. The exact solution by Onsager has
inspired several developments in various other fields. Some of these are also
briefly mentioned.
