  This is the second of a series of papers surveying some small part of the
remarkable work of our friend and colleague Nigel Kalton. We have written it as
part of a tribute to his memory. It does not contain new results. One of the
many topics in which Nigel made very significant and profound contributions
deals with commutators in interpolation theory. It was our great privilege to
work with him on one of his many papers about this topic. Our main purpose here
is to offer} an introduction to that paper: A unified theory of commutator
estimates for a class of interpolation methods. Adv. Math. 169 (2002), no. 2,
241--312. We sketch the theory of interpolation spaces constructed using
pseudolattices which was developed in that paper and which enables quite
general formulation of commutator theorems. We seek to place the results of
that paper in the general context of preceding and subsequent research on this
topic, also indicating some applications to other fields of analysis and
possible directions for future research.
