Although the first pyridazine was obtained in 1886, this heterocycle was not thoroughly investigated such isomers as pyrimidine and pyrazine especially in the field of drug development because pyridazine derivatives do not occur as natural products. Recently medicinal chemists have an growing interest in the pyridazine derivatives, since many Pyridazine derivatives were found to possess various potential therapeutic activities. In this paper sixty-eight pyridazine derivatives, which are already introduced as medicines or are being developed as drugs were classified according to their pharmacological activities, reviewed since 1955 and the relationship of structure-activities was discussed.