The article discusses issues related to the interpretation of sources – historical materials and documents of the XVII – XVIII century – in the context of the problems dealing with the correct choice of tempo and with the practical realization of the rhythmic conventions present in notation of early music. A brief review of modern scientific literature is made in this subject area. It is being noted that the scholars of the XX – XXI centuries applied different, most often mutually exclusive research methods. The authors of this paper emphasize the negative impact of various ‘brief introductions’ to the interpretation of early music on modern performing practice. In conclusion: even the most authoritative and solidly–founded scientific publications give either an incomplete, unconvincing, or an entirely erroneous picture of the musical performing arts practiced in the distant past. Since the middle of the last century, researchers (mostly American) have studied, translated and presented in the form of dissertations practically all the most reputable treatises of masters who wrote in the distant past. Such qualifying works, often provided with useful practical materials, contain important information that is relevant for both historians and theorists of musical art, as well as for practicing musicians. In our country, translation of sources with comments still cannot be a subject of a candidate or doctoral qualification work, which is obviously one of the atavisms of the previous Soviet system of certification of scholarly personnel and directly hinders the development of scientific knowledge in the field of historical musicology. Further, the article states that there are significant discrepancies and direct contradictions in the historical sources themselves – treatises, dictionaries and prefaces to music publications. The main indicators of the tempo of performance – signs of meter – in theory initially formed a hierarchically organized (stratified) system. However, the processes of convergence of various styles and aesthetic principles in XVIIIth century Western Europe consistently destroyed this system. Italian tempo terms played a significant part here, which, in their turn, were differently understood and interpreted by the authors of early music manuals. Thus, there had been no single, unified and accepted system for determining the tempo of performance in Western Europe of the Baroque era. However, a comparative analysis of descriptions of various precise and relatively accurate pre metronomic methods of recording the tempo of performance reveals some general trends in this area that go beyond regional and national traditions.