  In the last ten years three main facts about the thermally pulsing asymptotic
giant branch (TP-AGB) have become evident: 1) the modelling of the TP-AGB phase
is critical for the derivation of basic galaxy properties (e.g. mass and age)
up to high redshift, with consequent cosmological implications; 2) current
TP-AGB calibrations based on Magellanic Cloud (MC) clusters come out not to
work properly for other external galaxies, yielding a likely TP-AGB
overestimation; 3) the significance of the TP-AGB contribution in galaxies,
hence their derived properties, are strongly debated, with conflicting claims
in favour of either a heavy or a light TP-AGB. The only way out of this
condition of persisting uncertainty is to perform a reliable calibration of the
TP-AGB phase as a function of the star's initial mass (hence age) over a wide
range of metallicity, from very low to super-solar values. In this context, I
will review recent advancements and ongoing efforts towards a physically-sound
TP-AGB calibration that, moving beyond the classical use of the MC clusters,
combines increasingly refined TP-AGB stellar models with exceptionally
high-quality data for resolved TP-AGB stars in nearby galaxies. Preliminary
results indicate that a sort of "TP-AGB island" emerges in the age-metallicity
plane, where the contribution of these stars is especially developed, embracing
preferentially solar- and MC-like metallicities, and intermediate ages (~ few
Gyr).
