  After the discovery of the BRST identities in 1974 Raymond spent some two
years mostly on instantons. In those years we had a small group at the Centre
Physique Th\'eorique in Marseille discussing the physics and mathematics of
instantons.
  The upshot of our discussions can be found in a set of lectures given by
Raymond in Erice in 1977 and a year later in a Physics Reports volume. I
present some recollections of that period; mostly how we were influenced by the
twistor approach. I discuss the Atiyah-Drinfeld-Hitchin-Manin (ADHM) method to
obtain instantons ("calorons") in thermal QCD following earlier work. The
building blocks are a prepotential and a gauge invariant propagator.
  Then I will give surprisingly simple results in terms of these building
blocks for the one loop vacuum response to a change in caloron parameters and
some physical consequences for the effective action, in particular screening
properties.
