  We present EMIR, a powerful near-infrared (NIR) camera and multi-object
spectrograph (MOS) installed at the Nasmyth focus of the 10.4 m GTC. EMIR was
commissioned in mid-2016 and is offered as a common-user instrument. It
provides spectral coverage of 0.9 to 2.5 $\mu m$ over a field of view (FOV) of
6.67x6.67 squared arcmin in imaging mode, and 6.67x4 squared arcmin in
spectroscopy. EMIR delivers up to 53 spectra of different objects thanks to a
robotic configurable cold slit mask system that is located inside the cryogenic
chamber, allowing rapid reconfiguration of the observing mask. The imaging mode
is attained by moving all bars outside the FOV and then leaving an empty space
in the GTC focal surface. The dispersing suite holds three large pseudo-grisms,
formed by the combination of high-efficiency FuSi ion-etched ruled transmission
grating sandwiched between two identical ZnSe prisms, plus one standard
replicated grism. These dispersing units offer the spectral recording of an
atmospheric window $J,H,K$ in a single shot with resolving powers of 5000,
4250, 4000, respectively for a nominal slit width of 0.6\arcsec, plus the
combined bands $YJ$ or $HK$, also in a single shot, with resolution of $\sim$
1000. The original Hawaii2 FPA detector, which is prone to instabilities that
add noise to the signal, is being replaced by a new Hawaii2RG detector array,
and is currently being tested at the IAC. This paper presents the most salient
features of the instrument, with emphasis on its observing capabilities and the
functionality of the configurable slit unit. Sample early science data is also
shown.
