Lepton Flavour Universality (LFU) is one of the fundamental properties of the Standard Model (SM): the three lepton generations should be identical except for their masses. Flavour changing neutral current processes such as $b\to s\ell^+\ell^-$ are ideal to look for hints of LFU violation: they have small SM amplitudes, allowing sizeable contributions from hypotetical new particles. Clean observables such as the ratio between $B\to X_s\ell\ell$ decays, with $\ell=e,\mu$ and where $X_s$ is a strange meson, have been measured with the LHCb detector hinting at deviations from LFU. Further work on similar decays, such as $B\to K\pi\pi\ell\ell$, will allow to corroborate or disprove these deviations.