Recent analyses of drug attrition rates reveal that a significant number of drug candidates fail in the later stage of clinical development owing to absorption, distribution, metabolism, elimination (ADME), and toxicity issues. Lead optimization in drug discovery, a process attempting to uncover and correct these defects of drug candidates, is highly beneficial in lowering the cost and time to develop therapeutic drugs by reducing drug candidate failures in development. At present, parallel synthesis combining with high-throughput screening has made it easier to generate highly potent compounds (i. e. , hits). However, to be a potential drug, a hit must have drug-like characteristics in addition to potency, which include optimal physicochemical properties, reasonable ph- macokinetic parameters, and good safety profiles. Therefore, research tools must be available in drug discovery to rapidly screen for compounds with favorable drug-like properties, and thus adequate resources can be directed to projects with high potential. Optimization in Drug Discovery: In Vitro Methods is a compilation of detailed experimental protocols necessary for setting up a variety of assays important in compound evaluation. A total of 25 chapters, contributed by many experts in their research areas, cover a wide spectrum of subjects including physicochemical properties, abso- tion, plasma binding, metabolism, drug interactions, and toxicity. A good pharmacokinetic profile has long been recognized as an imp- tant drug-like characteristic. Pharmacokinetic parameters are affected by many properties of drug molecules such as physicochemical nature, abso- tion, metabolic stability, and so on.