We review the mechanisms which are thought to provide steady heating o
f chromospheres and coronae. It appears now fairly well established th
at nonmagnetic chromospheric regions of late-type stars are heated by
shock dissipation of acoustic waves which are generated in the stellar
surface convection zones. In the case of late-type giants there is ad
ditional heating by shocks from pulsational waves. For slowly rotating
stars, which have weak or no magnetic fields, these two are the domin
ant chromospheric heating mechanisms. Except for F-stars, the chromosp
heric heating of rapidly rotating late-type stars is dominated by magn
etic heating either through MHD wave dissipation (AC mechanisms) or th
rough magnetic field dissipation (DC mechanisms). The MHD wave and mag
netic field energy comes from fluid motions in the stellar convection
zones. Waves are also generated by reconnective events at chromospheri
c and coronal heights. The high-frequency part of the motion spectrum
leads to AC heating, the low frequency part to DC heating. The coronae
are almost exclusively heated by magnetic mechanisms. It is not possi
ble to say at the moment whether AC or DC mechanisms are dominant, alt
hough presently the DC mechanisms (e.g., nanoflares) appear to be the
more important. Only a more detailed study of the formation of and the
dissipation in small-scale structures can answer this question. The X
-ray emission in early-type stars shows the presence of coronal struct
ures which are very different from those in late-type stars. This emis
sion apparently arises in the hot post-shock regions of gas blobs whic
h are accelerated in the stellar wind by the intense radiation field o
f these stars.