An introduction to Reshetnyak's theory of subharmonic distances

The aim of the present text is to provide some basics around Reshetnyak’s theory of subharmonic distances, together with an overview of the main results. This text is intended to be a complement to the English translation of \cite{R1954}, \cite{Res1959}, \cite{R60I}, \cite{R60II}, \cite{R61}, \cite{R61b}, \cite{R62}, \cite{R63}, \cite{R63III} and \cite{huber}. They all will be published in a same volume \cite{livre}. While subharmonic distances are often confused with distances of bounded curvature, we present them as a complete autonomous theory. In turn, there is no specific prerequisite for the present text.