Rare $B$ meson decays are powerful probes for testing the Standard Model (SM). Recent measurements of several observables in $b\to s \mu\mu$ transitions have revealed tensions with the corresponding SM predictions. These tensions — or anomalies — have a significance above $3\sigma$. However, the theoretical predictions in these decays are contaminated by difficult-to-calculate QCD contributions, namely the non-local hadronic matrix elements. Understanding these contributions is of paramount importance to probe the SM and to clarify whether the anomalies are due to New Physics or to underestimated theoretical uncertainties.
