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The trilinear self-coupling can be measured directly using theHiggs-boson-pair production cross section, or indirectly through themeasurement of single-Higgs-boson production and decay modes. In fact, atnext-to-leading order in electroweak interaction, the Higgs-decay partialwidths and the cross sections of the main single-Higgs production processesdepend on the Higgs-boson self-coupling via weak loops. Measurements of$\kappa_\lambda$, i.e. the rescaling of the trilinear Higgs self-coupling, arepresented in this dissertation. Results are obtained exploiting proton-protoncollision data from the Large Hadron Collider at a centre-of-mass energy of 13TeV recorded by the ATLAS detector in 2015, 2016 and 2017, corresponding to aluminosity of up to 79.8 fb$^{-1}$. Constraints on the Higgs self-coupling arepresented 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) toimprove the sensitivity on $\kappa_\lambda$. Under the assumption that newphysics affects only the Higgs-boson self-coupling, the combined H+HH best-fitvalue 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 Modelcouplings. The final results of this thesis provide the most stringentconstraint on $\kappa_\lambda$ from experimental measurements to date.

Measurement of Higgs-boson self-coupling with single-Higgs and double-Higgs production channels