  Radio frequency (RF) superconductivity has become a key technology for many
modern particle accelerators. One of its most salient features of this
technology is the ability of superconducting RF cavities to deliver high
accelerating gradients in continuous-wave and long-pulse modes of operation.
However, reaching the current state of the technology was not an easy fit. Over
many years scientists and engineers had to overcome several serous performance
limitations. In this paper, I attempt to the best of my knowledge to trace the
history of accelerating gradients evolution in the field of superconducting
radio frequency. I will restrict the scope to primary innovations along with
some of the ensuing developments in developing cavities made of bulk niobium.
But I will not cover all the many applications and findings over the subsequent
decades of progress that were based on the primary discoveries and inventions.
I will also not cover a number of other important topics in the history of
cavity developments, such as the drive for higher Q values, or the push for
lower cavity costs via Nb/Cu cavities or large grain Nb cavities.
