In the class action context, "ascertainability"—a judicially created common law doctrine—is an implicit requirement within Rule 23 of the Federal Rules of Civil Procedure (which addresses class actions) that requires that class members be identifiable. "Historically, it referred only to the requirement that a class be defined with reference to objective criteria." Presently, the Third Circuit stands alone among the circuit courts to have both carefully considered the issue and adopted an additional ascertainability requirement, lacking support in the text of Rule 23. The Third Circuit view is that proponents of class certification "must [also] show a reliable, administratively feasible' mechanism to identify class members." Conversely, the Second, Sixth, Seventh, Eighth, and Ninth Circuits have expressly considered, and rejected, this approach.
Most recently, on July 7, 2017, a unanimous Second Circuit Panel in In re Petrobras Securities Litigation joined the "growing consensus" of circuit courts—believing that Rule 23's implied "ascertainability doctrine" requires no more than "that a class be defined using objective criteria that establish a membership with definitive boundaries." Breaking from the Third Circuit, the Petrobras Panel held that the proper ascertainability requirement calls only for class plaintiffs to undertake a "modest" inquiry of determining "whether a proposed class is defined using objective criteria that establish membership with definite boundaries." The Panel underscored the simplicity of this burden at the class certification stage, noting that "identifiable does not mean identified; ascertainability does not require a complete list of class members at the certification stage." Nor does the inquiry concern itself with "the plaintiffs' ability to offer proof of membership under a given class definition." Instead, the inquiry is satisfied in the Second Circuit unless the "proposed class definition is indeterminate in some fundamental way," such that there would be no "clear sense of who is suing who about what."
The Sixth Circuit likewise has rejected the Third Circuit's approach, stating in Rikos v. Procter & Gamble Co. that it saw "no reason to follow [the Third Circuit's decision in] Carrera, particularly given the strong criticism it has attracted from other courts."
The Seventh Circuit also has "decline[d] to follow th[e Third Circuit's] path," observing in Mullins v. Direct Digital, LLC that "[n]othing in Rule 23 mentions or implies this heightened requirement." To the contrary, the heightened requirement would effectively bar "class actions where class treatment is often most needed: involving relatively low-cost goods or services, where consumers are unlikely to have documentary proof of purchase." Instead, in the Seventh Circuit, objective criteria for, rather than absolute proof of, class membership is all that is required by Rule 23.
Perhaps offering an even stronger repudiation of the Third Circuit's requirement, the Eight Circuit has consistently "not addressed ascertainability as a separate, preliminary requirement" under Rule 23. While the Ninth Circuit likewise recently concluded that since "the language of Rule 23 does not impose a freestanding administrative feasibility prerequisite to class certification," it would "decline to interpose an additional hurdle into the class certification process."
It may be, however, that the Third Circuit has begun signaling an eventual departure from its prior decisions. In its 2015 decision in Byrd v. Aaron's, Inc., Judge Rendell explicitly wrote in a concurring opinion that it may well be time for the circuit to "retreat from [its] heightened ascertainability requirement." And in the Third Circuit's 2017 City Select decision, Judge Fuentes went so far as to state that "[w]e should join the Second, Sixth, Seventh, and Ninth Circuits in rejecting our added ascertainability requirement." Judge Fuentes concurred in the Judgment only inasmuch as he agreed that "until we revisit this issue as a full Court or it is addressed by the Supreme Court or the Advisory Committee on Civil Rules," stare decisis mandated that the circuit must "continue to administer the ascertainability requirement in a way that contravenes the purpose of Rule 23, and, in [his] view, disserves the public." Only time will tell if the circuit disagreement is meant to come to a close.
 See City Select Auto Sales Inc. v. BMW Bank of N. Am. Inc., 867 F.3d 434, 443 (3d Cir. 2017) (Fuentes, J., concurring).
 Id. (Fuentes, J., concurring).
 As the Third Circuit recently observed in City Select, the Fourth Circuit has applied an iteration of the Third Circuit's administrative feasibility requirement, but did so "without analyzing the adoption of this second requirement" at all, while the Eleventh Circuit has applied the requirement but only "in unpublished opinions." 867 F.3d 434, 443 n.3 (Fuentes, J. concurring) (citing EQT Prod. Co. v. Adair, 764 F.3d 347, 358 (4th Cir. 2014)).
 City Select, 867 F.3d at 443 (Fuentes, J., concurring) (internal citations and quotations omitted).
 862 F.3d 250, 265 (2d Cir. 2017).
 Id. at 266.
 799 F.3d 497, 525 (6th Cir. 2015), cert. denied, 136 S. Ct.1493 (2016).
 795 F.3d 654, 658 (7th Cir. 2015), cert. denied, 136 S. Ct. 1161 (2016).
 Sandusky Wellness Ctr., LLC v. Medtox Sci., Inc., 821 F.3d 992, 996 (8th Cir. 2016).
 Briseno v. ConAgra Foods, Inc., 844 F.3d 1121, 1124-25 (9th Cir.), cert. denied sub nom. ConAgra Brands, Inc. v. Briseno, 138 S. Ct. 313 (2017).
 784 F.3d 154, 177 (3d Cir. 2015) (Rendell, J., concurring).
 City Select, 867 F.3d at 448 (Fuentes, J., concurring).
 Id. at 448-49 (Fuentes, J., concurring).
*Gary Smith is an Associate in the Philadelphia office.