More than fifty years ago, Calderón introduced commutators and the Cauchy integral on Lipschitz curves, as part of his program in PDE. He asked whether these natural operators satisfy the same $L^p$ estimates as the classical singular integrals of convolution type. In spite of serious efforts by the harmonic analysis community, these problems resisted solution until about mid 80's, when "Calderón Program" has finally been completed.

The aim of the talk is not only to answer the question from the title, but also to explain why these ideas have had (and continue to have) such a profound impact in analysis and non-linear PDE.