The Sunyaev-Zeldovich effect is also an important probe of the cluster
medium. The scattering of the 2.7K background radiation by the hot
gas in the cluster gives a negative brightness signal proportional to
the integral of 
where 
is the electron density, 
is
the temperature of the hot, thermal gas, and 
is the path through
the cluster. This gives us an important second integral in addition to
the X-ray free-free and line emission through the cluster. This allows
one to construct much more complex, realistic models of the cluster
density and temperature than is possible using just the x-ray
observations. If clusters prove not to be too pathological, it also
allows a direct distance measurement which potentially can yield both
and 
from several such measurements.
Measuring the Sunyaev-Zeldovich effect is at the limit of the current VLA's capabilities. The enhanced VLA will make such work straightforward, especially in the cores of dense clusters. Other instruments, e.g., the MMA and the specialized array being proposed by Caltech, may be more sensitive to the Sunyaev-Zeldovich effect, however. Even so, the VLA will still serve the critical rôle of determining the distribution of confusing sources which must be subtracted from lower resolution images. The contribution of any radio halo emission in a target cluster must also be imaged as a function of frequency and, if necessary, corrected for. The enhanced VLA will be unparalleled for this. The VLA cannot now image fast enough or at the right frequencies for this task. Without the improvements to the VLA, whether or not the other instruments are built, the Sunyaev-Zeldovich measurements will not reach their full promise.