Gives a parameter-free universal similarity distance, the normalized compression distance (NCD), shows that it approximates optimality and explains how it can be used to extract a hierarchy of clusters from arbitrary domains with unknown features.

The Kolmogorov complexity of an object such as a piece of text is a measure of the computational resources needed to specify the object.

Argues that compressing, or equivalently, modeling natural language text is "AI-hard".

The intention of this prize is to encourage development of intelligent compressors/programs.