However, the rapid and continued increase of large-and ultra-large libraries has renewed the interest of the scientific community to generate/implement methods to handle and use for practical applications the largeand ultra-large chemical space associated with the newly generated compound libraries. Hence, the chemical space concept continues to be of significant interest to study the very large chemical libraries. There are several reviews addressing the concept of chemical space, covering different aspects such as enumeration of chemical compounds using de novo design, calculation of molecular descriptors, progress on visualization methods with emphasis on publicly available tools, web servers to explore the chemical space of chemical libraries, applications to study structure-property relationships. A recent development in this area is the chemical library networks. This development is further elaborated in Section 5.