  Some preliminaries and basic facts regarding unbounded Wiener-Hopf operators
(WH) are provided. WH with rational symbols are studied in detail showing that
they are densely defined closed and have finite dimensional kernels and
deficiency spaces. The latter spaces as well as the domains and ranges are
explicitly determined. A further topic concerns semibounded WH. Expressing a
semibounded WH by a product of a closable operator and its adjoint this
representation allows for a natural self-adjoint extension. It is shown that it
coincides with the Friedrichs extension. Polar decomposition gives rise to a
Hilbert space isomorphism relating semibounded WH to singular integral
operators of a well-studied type based on the Hilbert transformation.
