This article presents the observation of four-top-quark ($\rm t\bar{t}t\bar{t}$) production in proton-proton collisions by the ATLAS and CMS experiments at the CERN LHC.
The analysis is performed using data samples collected by both experiments separately at a center-of-mass energy of 13 TeV during 2016-2018
and corresponding to an integrated luminosity of 140 $\rm fb^{-1}$ and 138 $\rm fb^{-1}$ by the ATLAS and CMS Collaborations respectively.
Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected.
Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds.
The observed (expected) significance of the measured $\rm t\bar{t}t\bar{t}$ signal with respect to the standard model (SM) background-only hypothesis
is 6.1 (4.3) standard deviations by the ATLAS and 5.6 (4.9) standard deviations by CMS experiment.
The $\rm t\bar{t}t\bar{t}$ production cross section is measured to be $\rm 22.5^{+6.6}_{-5.5}$ fb and $\rm 17.7^{+4.4}_{-4.0}$ fb by the ATLAS and CMS experiment respectively,
in agreement with the available standard model predictions.
Data are also used by the ATLAS experiment to set limits on the three-top-quark production cross section, being an irreducible background not measured previously,
and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect $\rm t\bar{t}t\bar{t}$ production.