  At temperatures above ~5 keV, the non-relativistic approximation used to
derive the classical thermal Sunyaev-Zel'dovich effect spectrum begins to fail.
When relativistic effects are included, the spectrum becomes
temperature-dependent. This leads to both a problem and an opportunity: a
problem, because when the temperature dependence is not accounted for the
Compton-y estimate is biased; and an opportunity, because it represents a new
way to measure the temperature of the intracluster medium independently of
X-ray observations. This work presents current results from investigating the
impact of relativistic effects on Planck cluster observations, and projections
for future measurements of cluster temperatures using the Atacama Large
Aperture Sub-millimetre Telescope.
