  The degrees of freedom (DoF) region of the 2-user multiple-antenna or MIMO
(multiple-input, multiple-output) interference channel (IC) is studied under
fast fading and the assumption of {\em delayed} channel state information (CSI)
wherein all terminals know all (or certain) channel matrices perfectly, but
with a delay, and each receiver in addition knows its own incoming channels
instantaneously. The general MIMO IC is considered with an arbitrary number of
antennas at each of the four terminals. Dividing it into several classes
depending on the relation between the numbers of antennas at the four
terminals, the fundamental DoF regions are characterized under the delayed CSI
assumption for {\em all} possible values of number of antennas at the four
terminals. In particular, an outer bound on the DoF region of the general MIMO
IC is derived. This bound is then shown to be tight for all MIMO ICs by
developing interference alignment based achievability schemes for each class. A
comparison of these DoF regions under the delayed CSI assumption is made with
those of the idealistic `perfect CSI' assumption where perfect and
instantaneous CSI is available at all terminals on the one hand and with the
DoF regions of the conservative `no CSI' assumption on the other, where CSI is
available at the receivers but not at all at the transmitters.
