  The trilinear self-coupling can be measured directly using the
Higgs-boson-pair production cross section, or indirectly through the
measurement of single-Higgs-boson production and decay modes. In fact, at
next-to-leading order in electroweak interaction, the Higgs-decay partial
widths and the cross sections of the main single-Higgs production processes
depend on the Higgs-boson self-coupling via weak loops. Measurements of
$\kappa_\lambda$, i.e. the rescaling of the trilinear Higgs self-coupling, are
presented in this dissertation. Results are obtained exploiting proton-proton
collision data from the Large Hadron Collider at a centre-of-mass energy of 13
TeV recorded by the ATLAS detector in 2015, 2016 and 2017, corresponding to a
luminosity of up to 79.8 fb$^{-1}$. Constraints on the Higgs self-coupling are
presented considering the most sensitive double-Higgs channels (HH),
$b\bar{b}\tau^+\tau^-$, $b\bar{b}\gamma\gamma$ and $b\bar{b}b\bar{b}$,
considering single-Higgs (H) production modes, $ggF$, $VBF$, $ZH$, $WH$ and
$t\bar{t}H$, together with $WW^*$, $ZZ^*$, $\tau^+\tau^-$, $\gamma \gamma$ and
$b\bar{b}$ decay channels, and combining the aforementioned analyses (H+HH) to
improve the sensitivity on $\kappa_\lambda$. Under the assumption that new
physics affects only the Higgs-boson self-coupling, the combined H+HH best-fit
value of the coupling modifier is: $\kappa_\lambda = 4.6^{+3.2}_{-3.8}$,
excluding values outside the interval $-2.3<\kappa_\lambda<10.3$ at 95%
confidence level. Results with less stringent assumptions are also provided,
decoupling the Higgs-boson self-coupling and the other Standard Model
couplings. The final results of this thesis provide the most stringent
constraint on $\kappa_\lambda$ from experimental measurements to date.
