Class Actions and Mass Arbitration in 2026: The Plaintiff and Defense Playbooks Every Litigator Needs

Madelyn Petersen, Senior Associate | Cohen Milstein Sellers & Toll PLLC

Madelyn Petersen is an Associate at Cohen Milstein Sellers & Toll PLLC, where her practice focuses on litigating class actions on behalf of consumers who have been misled, deceived, or harmed by large corporations. Working across the firm’s Consumer Protection and Mass Torts practices, she advances cases that hold corporate actors accountable for deceptive and exploitative business practices, with the goal of fashioning a fairer consumer marketplace and a more just economic system.

• Education & Credentials

Madelyn earned her Juris Doctor from Harvard Law School in 2019 and her Bachelor of Arts from the University of Nebraska-Lincoln in 2016. She is admitted to practice in the District of Columbia.

• Recognition & Leadership

While at Harvard Law School, Madelyn served as Managing Editor of the Harvard Journal of Law and Gender and as Online Content Editor for the Harvard Civil Rights-Civil Liberties Law Review. She was also a board member of the Harvard Prison Legal Assistance Project, reflecting her early commitment to public-interest legal service.

• Professional Involvement

During law school, Madelyn participated in Harvard Law School’s International Human Rights Clinic and held legal internships with three leading public-interest organizations: the Corporate Accountability Lab, the Advancement Project, and Oxfam America. These experiences shaped her approach to corporate accountability litigation and her focus on systemic remedies for consumer harm.

• Experience

Madelyn joined Cohen Milstein after serving as a Law Fellow at the firm from 2021 to 2022, where she worked across multiple practice groups on individual and class action matters at both the district and appellate levels. Prior to her fellowship, she clerked for the Honorable William P. Dimitrouleas of the United States District Court for the Southern District of Florida from 2019 to 2021. Her current docket includes certified consumer class actions against the U.S. Department of State, General Motors, Meta Platforms, GreenSky, Siemens, and Ford Motor Company.

Collin Vierra, Partner | Eimer Stahl LLP

Collin Vierra is a Partner at Eimer Stahl LLP, where his nationwide practice focuses on complex trial and appellate litigation, commercial and consumer mass arbitration, and counseling. His work spans data privacy and AI, discrimination, products liability, commercial disputes, antitrust and unfair competition, environmental law, and government regulation, and clients trust him with their most high-stakes and cutting-edge disputes.

• Education & Credentials

Collin earned his Juris Doctor from Stanford Law School, his Master of Arts in Economics from Stanford University (where he was a Gregory Terrill Cox Fellow in the John M. Olin Program in Law and Economics), and his Bachelor of Science in Engineering from the Massachusetts Institute of Technology (MIT), where he was a member of the Pi Tau Sigma Mechanical Engineering Honors Society and an engineering apprentice in the Pappalardo Laboratory. He is licensed to practice in California, Texas, and Tennessee (Practice Pending Admission), and is admitted before multiple U.S. District Courts and the Second, Seventh, Eighth, and Ninth Circuit Courts of Appeals.

• Recognition & Leadership

Collin chairs Eimer Stahl’s Mass Arbitration Practice Group and co-chairs the firm’s Data Privacy and AI Practice Group. He has been recognized by Benchmark Litigation as a 40 & Under honoree in 2024 and 2025, named a Law.com/The Recorder Lawyer on the Fast Track in 2025, recognized by Legal 500 as a Recommended Lawyer in General Commercial Disputes (noting his “particular prowess in mass arbitration defense”), identified by Top Verdict as having obtained one of the top verdicts in California in 2024, and named to Best Lawyers: Ones to Watch in America for 2026.

• Professional Involvement

Collin is a featured speaker on mass arbitration and complex litigation issues, having presented to numerous institutions including the American Bar Association, the Federalist Society, the U.S. Chamber Litigation Center, Stanford Law School, MassArbCon, the Association of Corporate Counsel, and the Data Privacy and Cyber Security ConfEx. His expert analysis has been published in Law360 and EimerStahl Insights, and he is a current member of the MIT Free Speech Alliance.

• Experience

Collin has helped companies respond to over a quarter of a million individual demands for arbitration across the social networking, consumer hardware, and consumer entertainment industries, obtaining tens of thousands of dismissals without settlement payment. He defends clients under BIPA, CIPA, CCPA, VPPA, and other privacy statutes, and has represented major companies in consumer class actions, MDLs, and investigations by the DOJ, FTC, SEC, FDA, and Attorneys General of more than 45 states.

SESSION 1 – Plaintiff-Side Rule 23 Certification Playbook | 1:00pm – 2:00pm

Build a certification record that survives commonality, predominance, and standing attacks. Structure Comcast-compliant damages models, draft ascertainable class definitions, and position your case for the unresolved LabCorp uninjured-member split across circuits.

BREAK | 2:00pm – 2:10pm

SESSION 2 – Defense-Side Class Certification Playbook | 2:10pm – 3:10pm

Deploy commonality attacks, TransUnion standing challenges, and Comcast damages-model assaults at every procedural stage. Calibrate Rule 23(f) petitions, draft arbitration agreements that survive unconscionability review, and manage mass arbitration fee exposure under the 2024 AAA and JAMS rules.