The contents and diversity analysis will depend on the compound representation, discussed in Section 2. In general, the representations can be grouped into two major categories: molecular scaffolds and structural fingerprints. The selection of the compound representation to study a chemical library gives rise to the definition of the "chemical space" and will depend on the goals of the research program. In this section, we briefly discuss the concepts of chemical space and methods used to study systematically the diversity, variety of chemical scaffolds, and diversity of fingerprints. Several reviews address the basics and early developments of the analysis of the contents and diversity analysis of chemical space of compound libraries . As Dunn et al. recently pointed out, the rapid growth in the number and size of chemical libraries required the development of computational methods to analyze the heterogeneousness of ultra-large chemical libraries .