It is contended that there are such things as discourse blocs, and that they are composed of discourse units glued together into a contextuated whole by bloc signals. There are three kinds of structures with which it is necessary to deal in order to discuss coherent discourse: the discourse bloc, the discourse unit, and the bloc signal. Characteristically, a discourse bloc has a stated or implied topic. Topic, whether stated or implied, has a set of characteristics which may be preliminarily identified. Topic is likely to be either definite or generic. While topic often occurs as a complete sentence, it may also occur as a left-dislocated sentence modifier. It is also possible for the topic to occur in direct object position or in the position of postponed subject when subject position is occupied by some mechanism for permitting the subject to be moved to the right of the verb. Four types of definiteness noted by Hawkins are: anaphoric, immediate, occurring in a larger context, and associative. It is likely that all four types also occur with respect to the definite topic. Topic also has a logical function and a discourse function. Implications for language teaching, particularly in the relatively new areas of scientific and technological discourse, are noted. (SW)