In Williamson v. Citrix Online, the Federal Circuit altered the threshold test for determining whether a functional claim limitation that does not use the term means is governed by the scope—narrowing rules of § 112(f). Before Williamson, there was a strong presumption that functional claim limitations without the word means were not governed by § 112(f). After Williamson, § 112(f) now governs all functional limitations without the word means that do not recite sufficient structure for performing the claimed function. A common, incrementalist interpretation of Williamson is that the alteration of the § 112(f) threshold test will only have a small impact on the law of functional claiming. This interpretation frames Williamson as a case that will simply move the needle on the quantitative question about structure in the threshold test: How much structure need be recited in a limitation to avoid § 112(f)? In contrast, this Essay argues that, at least for software limitations in particular, Williamson will have a revolutionary impact on the law of functional claiming. Williamson will force the Federal Circuit to formulate a new answer to the more fundamental definitional question about software's structure: What constitutes structure in a software invention in the first place? Williamson demands a revolution in the definition of software's structure because the Federal Circuit's pre—Williamson doctrine that software's structure is an algorithm cannot do the work that Williamson requires. Although the concept of an algorithm can define corresponding structure in a specification in the course of construing the scope of a limitation that is known to be governed by § 112(f), it cannot identify structure in a claim limitation in the course of the threshold determination of whether or not a limitation is governed by § 112(f). After demonstrating that Williamson demands a revolution in software's structure under § 112(f), this Essay