  We present the wide field slitless spectroscopy mode of the NIRISS instrument
on the James Webb Space Telescope. This mode employs two orthogonal
low-resolution (resolving power $\approx 150$) grisms in combination with a set
of six blocking filters in the wavelength range 0.8 to $2.3\,\mu$m to provide a
spectrum of almost every source across the field-of-view. When combined with
the low background, high sensitivity and high spatial resolution afforded by
the telescope, this mode will enable unprecedented studies of the structure and
evolution of distant galaxies. We describe the performance of the as-built
hardware relevant to this mode and expected imaging and spectroscopic
sensitivity. We discuss operational and calibration procedures to obtain the
highest quality data. As examples of the observing mode usage, we present
details of two planned Guaranteed Time Observations programs: The Canadian
NIRISS Unbiased Cluster Survey (CANUCS) and The NIRISS Survey for Young Brown
Dwarfs and Rogue Planets.
