  In this Chapter we review the challenges of, and opportunities for, 3D
spectroscopy, and how these have lead to new and different approaches to
sampling astronomical information. We describe and categorize existing
instruments on 4m and 10m telescopes. Our primary focus is on grating-dispersed
spectrographs. We discuss how to optimize dispersive elements, such as VPH
gratings, to achieve adequate spectral resolution, high throughput, and
efficient data packing to maximize spatial sampling for 3D spectroscopy. We
review and compare the various coupling methods that make these spectrographs
``3D,'' including fibers, lenslets, slicers, and filtered multi-slits. We also
describe Fabry-Perot and spatial-heterodyne interferometers, pointing out their
advantages as field-widened systems relative to conventional, grating-dispersed
spectrographs. We explore the parameter space all these instruments sample,
highlighting regimes open for exploitation. Present instruments provide a foil
for future development. We give an overview of plans for such future
instruments on today's large telescopes, in space, and in the coming era of
extremely large telescopes. Currently-planned instruments open new domains, but
also leave significant areas of parameter space vacant, beckoning further
development.
