Antimicrobial chemotherapy has conferred huge benefits on human health. A variety of microorganisms were elucidated to cause infectious diseases in the latter half of the 19th century. Thereafter, antimicrobial chemotherapy made remarkable advances during the 20th century, resulting in the overly optimistic view that infectious diseases would be conquered in the near future. However, in response to the development of antimicrobial agents, microorganisms that have acquired resistance to drugs through a variety of mechanisms have emerged and continue to plague human beings. In Japan, as in other countries, infectious diseases caused by drugresistant bacteria are one of the most important problems in daily clinical practice. In the current situation, where multidrug-resistant bacteria have spread widely, options for treatment with antimicrobial agents are limited, and the number of brand new drugs placed on the market is decreasing. Since drug-resistant bacteria have been selected by the use of antimicrobial drugs, the proper use of currently available antimicrobial drugs, as well as efforts to minimize the transmission and spread of resistant bacteria through appropriate infection control, would be the first step in resolving the issue of resistant organisms.