Clinical trial is the most important step in new drug development, for only through which the efficacy and safety of a new drug can be explored and proved. Phases of clinical trials at present have been established through a series of changes with improvements of relevant regulations and laws following drug tragedies such as thalidomide baby in Europe around 1960. But it was not until the Kefauver-Harris Amendments was announced in 1962 and Investigational New Drug(IND) and New Drug Application(NDA) enforced in 1963 that regulations for safety and efficacy of a new drug and concrete guidelines for clinical trial enacted. FDA had propelled the development of clinical trial in respect of science and ethics since Helsinki proclamation in 1964 and obligated Good Clinical Practice(GCPs). But since International Conference on Harmonization(ICH), a movement for international standardization of new drug development process in 1992, conceptual change in preclinical, clinical data and clinical stage of new drug development goes on. Thus, we tried to help understand the new drug development process and clinical trial by reviewing them and to investigate the condition in Korea.