  About 90% of baryons in the universe have thus far escaped direct
observation. This is known as the {\it missing baryon problem}. The Sunyaev
Zel'dovich effect (SZ effect) has the potential to directly measure the state
of the majority of these missing baryons. The next generation CMB experiments
such as AMIBA will provide an unbiased sample of the intergalactic medium
through the SZ effect. The existing and upcoming simulations and analytical
studies provide a quantitative understanding of the SZ effect. All these make
the SZ effect a powerful baryon probe. We present an overview of this probe
from both phenomelogical and theoretical aspects.
