Rosen’s Rule states that rendaku is predictable when at least one element in a compound is three moras or longer. The rule is of importance for two reasons. Firstly, it adds further to the evidence afﬁrming the role of the foot in Japanese. Secondly, the application of Rosen’s Rule allows us to ﬂag a given morpheme for rendaku immunity (never undergoing rendaku) or its opposite, rendaku submission (always undergoing rendaku). However, recent work on Rosen’s Rule has shown that its psychological reality is unproven and, due to inconsistent deﬁnitions in Rosen’s work, scholarly interpretation of the Rule has not yet reached a consensus. Harnessing the Rendaku Database, the Rule is subjected to a thoroughgoing analysis, the outcome of which is that Rosen’s Rule is judged to be highly robust, with an accuracy level in the area of 90%.