Present understanding of physical aspects of combustion in fires is summarized in this paper. Following a brief description made on present achievements and remaining problems in fire physics, a comprehensible interpretation is attempted on basic physical phenomena controlling combustion in fires, such as heat and mass transfer, behavior of heat sources, and fire induced flows. Also, a concept of propagation of states is introduced to characterize the fire phenomena. It is emphasized that understanding of fundamental combustion phenomena in fires is essential to enhance the abilities of fire protection engineers.