  Variable sources probe a wide range of astrophysical phenomena. We present a
catalog of over ten million variable source candidates found in Data Release 1
(DR1) of the Zwicky Transient Facility (ZTF). We perform a periodicity search
up to a frequency of 160 day^-1, and we classify the light curves into erratic
and smooth variables. We also present variability indicators and the results of
a periodicity search, up to a frequency of 5 day^-1, for about 1 billion
sources in the ZTF-DR1 light curve database. We present several new
short-period (<90 min) candidates, and about 60 new dwarf nova candidates,
including two candidate eclipsing systems. Both the 10 million variables
catalog and ~1 billion source catalog are available online in catsHTM format.
