  The recent association between IC-170922A and the blazar TXS0506+056
highlights the importance of real-time observations for identifying possible
astrophysical neutrino sources. Thanks to its near-100\% duty cycle, 4$\pi$
steradian field of view, and excellent sensitivity over many decades of energy,
IceCube is well suited both to generate alerts for follow-up by other
instruments and to rapidly follow up alerts generated by other instruments.
Detection of neutrinos in coincidence with transient astrophysical phenomena
serves as a smoking gun for hadronic processes and supplies essential
information about the identities and mechanisms of cosmic-ray accelerators. In
2016, the IceCube Neutrino Observatory established a pipeline to rapidly search
for neutrinos from astrophysical transients on timescales ranging from a
fraction of a second to multiple weeks. Since then, 67 dedicated analyses have
been performed searching for associations between IceCube neutrinos and
astrophysical transients reported by radio, optical, X-ray, and gamma-ray
instruments in addition to searching for lower energy neutrino signals in
association with IceCube's own high-energy alerts. We present the event
selection, maximum likelihood analysis method, and sensitivity of the IceCube
real-time pipeline. We also summarize the results of all follow-up analyses to
date.
