  We describe the sources of stray light and thermal background that affect
JWST observations, report actual backgrounds as measured from commissioning and
early-science observations, compare these background levels to prelaunch
predictions, estimate the impact of the backgrounds on science performance, and
explore how the backgrounds probe the achieved configuration of the deployed
observatory. We find that for almost all applications, the observatory is
limited by the irreducible astrophysical backgrounds, rather than scattered
stray light and thermal self-emission, for all wavelengths lambda < 12.5
micron, thus meeting the level 1 requirement. This result was not assured given
the open architecture and thermal challenges of JWST, and it is the result of
meticulous attention to stray light and thermal issues in the design,
construction, integration, and test phases. From background considerations
alone, JWST will require less integration time in the near-infrared compared to
a system that just met the stray-light requirements; as such, JWST will be even
more powerful than expected for deep imaging at 1-5 micron. In the
mid-infrared, the measured thermal backgrounds closely match prelaunch
predictions. The background near 10 micron is slightly higher than predicted
before launch, but the impact on observations is mitigated by the excellent
throughput of MIRI, such that instrument sensitivity will be as good as
expected prelaunch. These measured background levels are fully compatible with
JWST's science goals and the Cycle 1 science program currently underway.
