  The ATLAS detector is installed in its experimental cavern at Point 1 of the
CERN Large Hadron Collider. During Run 2 of the LHC, a luminosity of
$\mathcal{L}=2\times 10^{34}\mathrm{cm}^{-2}\mathrm{s}^{-1}$ was routinely
achieved at the start of fills, twice the design luminosity. For Run 3,
accelerator improvements, notably luminosity levelling, allow sustained running
at an instantaneous luminosity of $\mathcal{L}=2\times
10^{34}\mathrm{cm}^{-2}\mathrm{s}^{-1}$, with an average of up to 60
interactions per bunch crossing. The ATLAS detector has been upgraded to
recover Run 1 single-lepton trigger thresholds while operating comfortably
under Run 3 sustained pileup conditions. A fourth pixel layer 3.3 cm from the
beam axis was added before Run 2 to improve vertex reconstruction and
$b$-tagging performance. New Liquid Argon Calorimeter digital trigger
electronics, with corresponding upgrades to the Trigger and Data Acquisition
system, take advantage of a factor of 10 finer granularity to improve
triggering on electrons, photons, taus, and hadronic signatures through
increased pileup rejection. The inner muon endcap wheels were replaced by New
Small Wheels with Micromegas and small-strip Thin Gap Chamber detectors,
providing both precision tracking and Level-1 Muon trigger functionality. Tile
Calorimeter scintillation counters were added to improve electron energy
resolution and background rejection. Upgrades to Minimum Bias Trigger
Scintillators and Forward Detectors improve luminosity monitoring and enable
total proton-proton cross section, diffractive physics, and heavy ion
measurements. These upgrades are all compatible with operation in the much
harsher environment anticipated after the High-Luminosity upgrade of the LHC
and are the first steps towards preparing ATLAS for the High-Luminosity upgrade
of the LHC. This paper describes the Run 3 configuration of the ATLAS detector.
