  The performance of the ATLAS Inner Detector alignment has been studied using
$pp$ collision data at $\sqrt{s} = 13$ TeV collected by the ATLAS experiment
during Run 2 (2015 to 2018) of the Large Hadron Collider (LHC). The goal of the
detector alignment is to determine the detector geometry as accurately as
possible and correct for time-dependent movements. The Inner Detector alignment
is based on the minimization of track-hit residuals in a sequence of
hierarchical levels, from global mechanical assembly structures to local
sensors. Subsequent levels have increasing numbers of degrees of freedom; in
total there are almost 750000. The alignment determines detector geometry on
both short and long timescales, where short timescales describe movements
within an LHC fill. The performance and possible track parameter biases
originating from systematic detector deformations are evaluated. Momentum
biases are studied using resonances decaying to muons or to electrons. The
residual sagitta bias and momentum scale bias after alignment are reduced to
less than $\sim$0.1 TeV$^{-1}$ and 0.9$\times10^{-3}$, respectively. Impact
parameter biases are also evaluated using tracks within jets.
