  For the initial mass range (140 < M < 260 Msun) stars die in a thermonuclear
runaway triggered by the pair-production instability. The supernovae they make
can be remarkably energetic (up to ~10^53 ergs) and synthesize considerable
amounts of radioactive isotopes. Here we model the evolution, explosion, and
observational signatures of representative pair-instability supernovae (PI SNe)
spanning a range of initial masses and envelope structures. The predicted light
curves last for hundreds of days and range in luminosity, from very dim to
extremely bright, L ~ 10^44 ergs/s. The most massive events are bright enough
to be seen at high redshift, but the extended light curve duration (~1 year) --
prolonged by cosmological time-dilation -- may make it difficult to detect them
as transients. An alternative approach may be to search for the brief and
luminous outbreak occurring when the explosion shock wave reaches the stellar
surface. Using a multi-wavelength radiation-hydrodynamics code we calculate
that, in the rest-frame, the shock breakout transients of PI SNe reach
luminosities of 10^45-10^46 ergs/s, peak at wavelengths ~30-170 Angstroms, and
last for several hours. We explore the detectability of PI SNe emission at high
redshift, and discuss how observations of the light curves, spectra, and
breakout emission can be used to constrain the mass, radius, and metallicity of
the progenitor.
