  The theoretical calculation of pseudo-scalar leptonic decay widths into an
invisible ALP, $M \to \ell\, \nu_\ell\, a$, is reviewed. Assuming generic
flavour-conserving ALP couplings to SM fermions and a generic ALP mass, $m_a$,
the latest experimental results for pseudo-scalar leptonic decays are used to
provide updated bounds on the ALP-fermion Lagrangian sector. Constrains on the
ALP-quark couplings obtained from these channels are not yet competitive with
the ones derived from FCNC processes, like $M\to P\,a$ decays. These leptonic
decays can, however, provide the most stringent model-independent upper bounds
on ALP-leptons couplings for $m_a$ in the (sub)-GeV range.
