In recent decades, alternatives to the established bubonic-plague theory have been presented as to the microbiologcal identity and mechanism(s) of spread of historical plague epidemics. In this monograph, the six important alternative theories are intensively discussed in the light of the historical sources, the central primary studies and standard works on bubonic plague and the alternative microbiological agents, insofar as they are testable. These seven theories are incompatible and at least six of them must be untenable. In the author's opinion, the arguments against the bubonic-plague theory and for all alternative theories are untenable. This monograph therefore also has been written also as a standard work on bubonic plague, giving a broad and in-depth presentation of the medical, epidemiological and historical evidence and the methodological tenets for identification of historical diseases by comparison with modern medical knowledge.