This report aims at characterizing and analyzing the flashover which is noted for the rapid change of environmental conditions accompanying with it but is now understood quite vaguely, and developing the methods and concepts for the evaluation on flashover, as a following up of BRI Research Paper No.83(1979) by the same author. This report consists of three parts, and the following problems are discussed in the respective parts.

Part-I	Method to judge whether flashover occurs or not
Part-II	Characterization of flashover in transient fire
Part-III	Behavior of combustibles in flashover

Of course a countermeasure such that flashover may never occur which is the main problem in Part-I is most desirable, but it may not be realistic for some sorts of building spaces. Out of the recognition on such problems, we discuss on the mathematical characterization of flashover in transient fire assuming its occurrence in Part-Il, and then discuss on the physical mechanism promoting flashover and some concepts useful for analyzing and evaluating flashover in Part-III on the basis of the results of Part-II.