  This is the first part of a possible monograph on the duality of Markov
processes. It contains a proof of Fitzsimmons' existence theorem of a moderate
Markov dual process relative to an excessive measure, m, together with the
necessary preliminary material. Then this is applied to prove the
correspondence between optional copredictable homogenous random measures and
sigma finite measures not charging m-exceptional sets again following
Fitzsimmons. The second part which may never be written would deal with duality
proper including results from, but not limited to, my joint paper with P. J.
Fitzsimmons"Potential Theory of Moderate Markov Dual Processes" which appeared
in Potential Anal.(2009) 31:275-310. Complete proofs of all results not
appearing in standard books are given with the one exception of Dellacherie's
result characterizing semipolar sets.
