  The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope
(JWST) offers the first opportunity to use integral-field spectroscopy from
space at near-infrared wavelengths. More specifically, NIRSpec's integral-field
unit can obtain spectra covering the wavelength range $0.6 - 5.3~\mu$m for a
contiguous 3.1 arcsec $\times$ 3.2 arcsec sky area at spectral resolutions of
$R \approx 100$, 1000, and 2700. In this paper we describe the optical and
mechanical design of the NIRSpec integral-field spectroscopy mode, together
with its expected performance. We also discuss a few recommended observing
strategies, some of which are driven by the fact that NIRSpec is a multipurpose
instrument with a number of different observing modes, which are discussed in
companion papers. We briefly discuss the data processing steps required to
produce wavelength- and flux-calibrated data cubes that contain the spatial and
spectral information. Lastly, we mention a few scientific topics that are bound
to benefit from this highly innovative capability offered by JWST/NIRSpec.
