We relate the classical Hodge filtration of a complex algebraic
variety to the ``Hodge'' decomposition of its cyclic homology.
The periodic cyclic homology of the variety agrees with the
Betti cohomology, and the tower of S-maps to cyclic homology
determines a filtration. For a smooth projective variety,
this is exactly the classical Hodge filtration.
In order to prove these results, it is necessary to set up the machinery
of cyclic homology for mixed complexes of sheaves. This is done
for all quasi-compact quasi-separated schemes over a base ring k.
In characteristic zero, this produces a finite decomposition of each
cyclic homology group.
This has appeared in K-theory, 12 (1997), 145-164.