Drug discovery is the process by which new drug candidates are identified to treat or prevent diseases. Drug discovery involves various stages: target identification, hits identification, lead generation and optimization, and drug candidate identification. First, molecular targets of the drugs, such as receptors, enzymes, and other functional proteins and nucleic acids, are identified using biochemical assays, genetic interactions, and computational methods. The next step is identifying hit, molecules that bind to the selected biological target. To do so, libraries of thousands of compounds, chemically synthesized or natural products, are screened using high-throughput assays. Hits that pass the primary screening are modified to generate leads that show pharmacological activity against the chosen target. These leads are optimized by combinatorial chemistry to improve their potency and target selectivity and reduce adverse effects. They are also tested for their pharmacokinetics and bioavailability. Such screening identifies a few drug candidates that are further tested for clinical applications.