  The physics of isolated plasma potential structures sustained by a deficit of
phase-space density on trapped orbits, commonly known as electron or ion holes,
is reviewed. The principles of their equilibria are explained and illustrated,
and contrasted with solitons. A literature review mostly prior to 2016
highlights the key historical developments of the field. Progress since,
especially in hole acceleration, stability, and multi-dimensional effects, is
summarized in more detail.
