  In the 1970s Alain Connes identified the appropriate notion of amenabilty for
von Neumann algebras, and used it to obtain a deep internal finite dimensional
approximation structure for these algebras. This structure is exactly what is
needed for classification, and one of many consequences of Connes' theorem is
the uniqueness of amenable II$_1$ factors, and later a complete classification
of all simple amenable von Neumann algebras acting on separable Hilbert spaces.
  The Elliott classification programme aims for comparable structure and
classification results for $C^*$-algebras using operator $K$-theory and traces.
The definitive unital classification theorem was obtained in 2015. This is a
combination of the Kirchberg--Phillips theorem and the large scale activity in
the stably finite case by numerous researchers over the previous 15--20 years.
It classifies unital simple separable amenable $C^*$-algebras satisfying two
extra hypotheses: a universal coefficient theorem which computes $KK$-theory in
terms of $K$-theory and a regularity hypothesis excluding exotic high
dimensional behaviour. Today the regularity hypothesis can be described in
terms of tensor products (Jiang-Su stability). These hypotheses are abstract,
and there are deep tools for verifying the universal coefficient theorem and
Jiang-Su stability in examples.
  This article describes the unital classification theorem, its history and
context, together with the new abstract approach to this result developed in
collaboration with Carri\'on, Gabe, Schafhauser and Tikuisis. This method makes
a direct connection to the von Neumann algebraic results, and does not need to
obtain any kind of approximation structure inside $C^*$-algebras en route to
classification. A companion survey will focus on the role of the Jiang-Su
stability hypothesis, and the associated work on "regularity."
