  Infrared tip-tilt sensors (IR TTSs) have been deployed on three different
laser guide star adaptive optics (AO) systems on three different telescopes.
These IR TTS benefit from the high-order loop PSF sharpening in the near
infrared, hence they provide a low tip-tilt residual and a good sky coverage.
Nevertheless, these IR TTS are challenging and their use in AO is limited. In
this paper, we outline existing IR TTS, provide on-sky performance results and
describe our experience using IR TTS and along with plans for the near future.
The second part of the paper deals with unresolved challenges for IR TTS. These
include algorithms and loop stability in the low Strehl regime, using IR TTSs
to measure higher-order modes and guiding on multiple guide stars with
different magnitudes.
