Virtual screening is a general approach devised to predict promising molecules, termed computational hits, that, upon experimental validation, have the potential to turn into lead molecules. Hits selected based on other features, such as physicochemical properties and smooth SAR, become leads ready to undergo optimization cycles. After intensive optimization, a drug-like molecule, termed a clinical candidate, might be considered for further preclinical and clinical development. This process can be extremely costly and timeconsuming and is bound to high attrition rates. Therefore, virtual screening could assist in hit identification at early preclinical stages. Iterative rounds of virtual screenings and scaffold-based analyses could then aid in hit expansion and lead optimization. In a recent example, Steadman et al. reported a dockingbased virtual screening to identify new inhibitors of Notum, a negative regulator of Wnt signaling. They screened several successful series and found the With the rise of large and ultra-large chemical databases, virtual screening has evolved as a natural way to exploit their contents and diversity. Besides a database to search in, virtual screening requires additional information, for example, the receptor's structure and a force field for docking scoring (example of a structure-based approach) or known ligands and a system for assessing similarity (example of a ligandbased approach). Throughout this section, we will focus on ligand-based virtual screening, in particular, similarity-based virtual screening. This approach typically includes using one or more bioactive molecules as queries or references to compare against the database. The compounds most similar to the reference molecules are the computational hits.[101] Measuring similarity is, nonetheless, dependent on the operational definition of similarity: it might use, for example, chemical fingerprints, physicochemical properties, pharmacophoric features, and even combined approaches.[102] Several strategies for performing ligandbased virtual screening exist, but there is no consensus on the best method for every case.