  Currently three imaging spectrometer architectures, tunable filter,
dispersive, and Fourier transform, are viable for imaging the universe in three
dimensions. There are domains of greatest utility for each of these
architectures. The optimum choice among the various alternative architectures
is dependent on the nature of the desired observations, the maturity of the
relevant technology, and the character of the backgrounds. The domain
appropriate for each of the alternatives is delineated; both for instruments
having ideal performance as well as for instrumentation based on currently
available technology. The environment and science objectives for the Next
Generation Space Telescope will be used as a specific representative case to
provide a basis for comparison of the various alternatives.
