  The Kepler conjecture asserts that no packing of congruent balls in
three-dimensional Euclidean space has density greater than that of the
face-centered cubic packing. In 1998, Sam Ferguson and I announced a
computer-assisted proof of this conjecture. Long delays in the refereeing
process sparked a project to give a formal proof of the Kepler conjecture,
which was completed in a large collaborative effort in 2014. This article gives
a critical reappraisal of that project.
