  Integrability is believed to underlie the AdS3/CFT2 correspondence with
sixteen supercharges. We elucidate the role of massless modes within this
integrable framework. Firstly, we find the dressing factors that enter the
massless and mixed-mass worldsheet S matrix. Secondly, we derive a set of
all-loop Bethe Equations for the closed strings, determine their symmetries and
weak-coupling limit. Thirdly, we investigate the underlying Yangian symmetry in
the massless sector and show that it fits into the general framework of Yangian
integrability. In addition, we compare our S matrix in the near-relativistic
limit with recent perturbative worldsheet calculations of Sundin and Wulff.
