  We review lattice results related to pion, kaon, $D$-meson, $B$-meson, and
nucleon physics with the aim of making them easily accessible to the nuclear
and particle physics communities. More specifically, we report on the
determination of the light-quark masses, the form factor $f_+(0)$ arising in
the semileptonic $K \to \pi$ transition at zero momentum transfer, as well as
the decay constant ratio $f_K/f_\pi$ and its consequences for the CKM matrix
elements $V_{us}$ and $V_{ud}$. Furthermore, we describe the results obtained
on the lattice for some of the low-energy constants of $SU(2)_L\times SU(2)_R$
and $SU(3)_L\times SU(3)_R$ Chiral Perturbation Theory. We review the
determination of the $B_K$ parameter of neutral kaon mixing as well as the
additional four $B$ parameters that arise in theories of physics beyond the
Standard Model. For the heavy-quark sector, we provide results for $m_c$ and
$m_b$ as well as those for the decay constants, form factors, and mixing
parameters of charmed and bottom mesons and baryons. These are the heavy-quark
quantities most relevant for the determination of CKM matrix elements and the
global CKM unitarity-triangle fit. We review the status of lattice
determinations of the strong coupling constant $\alpha_s$. We consider nucleon
matrix elements, and review the determinations of the axial, scalar and tensor
bilinears, both isovector and flavor diagonal. Finally, in this review we have
added a new section reviewing determinations of scale-setting quantities.
