Numerical characterization of molecular structure is a first step in many computational analysis of chemical
structure data. These numerical representations, termed descriptors, come in many forms, ranging from simple atom
counts and invariants of the molecular graph to distribution of properties, such as charge, across a molecular surface. In
this article we first present a broad categorization of descriptors and then describe applications and toolkits that can be
employed to evaluate them. We highlight a number of issues surrounding molecular descriptor calculations such as
versioning and reproducibility and describe how some toolkits have attempted to address these problems.