  Classic solar models put the Chromosphere-Corona Transition Region (CCTR} at
$\sim2$Mm above the $\tau_{5000} = 1$ level, whereas rMHD models place it in a
wider range of heights. Observational verification is scarce. We review and
discuss recent results from various instruments and spectral domains. In SDO
and TRACE images spicules appear in emission in the 1600, 1700 and 304 A bands
and in absorption in the EUV bands; the latter is due to photo-ionization of H
and He I. At the shortest available AIA wavelength and taking into account that
the photospheric limb is $\sim0.34$Mm above the $\tau_{5000}=1$ level, we found
that CCTR emission starts at $\sim3.7$Mm; extrapolating to $\lambda=0$, where
there is no chromospheric absorption, we deduced a height of $3.0\pm 0.5$Mm,
above the value of 2.14Mm of the Avrett & Loeser model. Another indicator of
the extent of the chromosphere is the height of the network structures. Height
differences produce a limbward shift of features with respect to their
counterparts in magnetograms. Using this approach, we measured heights of
$0.14\pm0.04$Mm (at 1700 A), $0.31\pm0.09$Mm (at 1600 A) and $3.31\pm 0.18$Mm
(at 304 A) for the center of the solar disk. A previously reported possible
solar cycle variation is not confirmed. A third indicator is the position of
the limb in the UV, where IRIS observations of the Mg II triplet lines show
that they extend up to $\sim2.1$Mm above the 2832 A limb, while AIA/SDO images
give a limb height of $1.4 \pm 0.2$Mm (1600 A) and $5.7\pm 0.2$Mm (304 A).
Finally, ALMA mm-$\lambda$ full-disk images provide useful diagnostics, though
not very accurate; values of $2.4\pm0.7$Mm at 1.26mm and $4.2\pm2.5$Mm at 3mm
were obtained. Putting everything together, we conclude that the average
chromosphere extends higher than homogeneous models predict, but within the
range of rMHD models.
