Toxicology is "the science of poisons"; more specifically the chemical and physical properties of poisons, their physiological or behavioral effects on living organisms, qualitative, and quantitative methods for their analysis and the development of procedures for the treatment of poisoning. Although the history of poisons dates to the earliest times, the study and the science of toxicology can be traced to Paracelsus (1493-1541) and Orfila (1757-1853). Modern toxicology is characterized by sophisticated scientific investigation and evaluation of toxic exposures. The 20th century is marked by an advanced level of understanding of toxicology. DNA and various biochemicals that maintain cellular functions were discovered. Our level of knowledge of toxic effects on organs and cells is now being revealed at the molecular level. This paper will review the historical progress of clinical and forensic toxicology by exploring analytical techniques in drug analysis, differing biological matrices, clinical toxicology, therapeutic drug management, workplace drug testing, and pharmacodynamic monitoring and pharmacogenetics.