In spite of numerous revisions from 1966 to present, the Cantabrian Substage of the Stephanian Stage (Pennsylvanian) was never properly defined as a chronostratigraphic unit. Defined and redefined at least three times, the Cantabrian lacks boundary stratotypes that correspond to clear and correlateable biochronological signals. Thus, instead of using a biochronological datum of well-established validity and utility, Cantabrian advocates have relied on ill-defined macrofloral assemblage zones and on lithostratigraphic boundaries to define the substage. As a result, the Cantabrian is demonstrably diachronous, even within Europe; indeed, the Cantabrian has proven to be unusable for correlations outside its type area in northern Spain. To resolve these problems, we recommend that the Cantabrian Substage be abandoned, and the Westphalian–Stephanian boundary be redefined at the major floral turnover that has been documented in the USA, western and central Europe, and in the Donets Basin. We further recommend that the bases of the Kasimovian Series, Stephanian Series, Missourian Series, and Upper Pennsylvanian Series all be aligned with this same floral turnover.