The onset of chromospheric activity appears at late-A and early-F stars
where theories predict atmospheres in radiative equilibrium
and shallow or non-existent convective zones.
The detection of Ly-α emission cores
in several A and F stars, first with the IUE satellite and then with
the HST, gives evidence for the presence of chromospheric layers
in these stars up to B-V=0.m19 (Catalano et al. 1986).
Semiempirical chromospheric models for Altair allowed us
(Freire et al. 1995) to explain the observed emission profiles
taking into account normal HI IS absorption.
However, due to the very high rotational velocity we analyzed
alternative hypotheses like the formation of Ly-α emissions
into a corotating expanding wind, but we ruled out this alternative
because we obtained inconsistent results. In addition, X-ray emission
(originated surely in a corona) strengthen
the presence of a chromosphere.
Here we place the problem of chromospheric activity of
late-A and early-F stars in the general context of the formation
of over-photospheric stellar layers, comparing
them with late-type star and solar cases.