  The main results presented in this paper provide a complete and explicit
description of all solutions to the left tangential operator Nevanlinna- Pick
interpolation problem assuming the associated Pick operator is strictly
positive. The complexity of the solutions is similar to that found in
descriptions of the sub-optimal Nehari problem and variation on the
Nevanlinna-Pick interpolation problem in the Wiener class that have been
obtained through the band method. The main techniques used to derive the
formulas are based on the theory of co-isometric realizations, and use the
Douglas factorization lemma and state space calculations. A new feature is that
we do not assume an additional stability assumption on our data, which allows
us to view the Leech problem and a large class of commutant lifting problems as
special cases. Although the paper has partly the character of a survey article,
all results are proved in detail and some background material has been added to
make the paper accessible to a large audience including engineers.
