Sonia Kuester Pfaffenroth is a partner in Sullivan & Cromwell’s Litigation Group and a member of the Antitrust Group. She represents clients in complex U.S. and international antitrust matters, including federal and state civil antitrust litigation, government investigations and cartel matters. Her practice also includes merger reviews, antitrust compliance and training, and day-to-day counseling on pricing, marketing, distribution, contracting, trade association participation and R&D collaborations. Sonia has particular experience representing life sciences and consumer products companies and has handled matters for clients in the technology, energy, manufacturing and automotive industries.
Government Service
Before returning to private practice, Sonia held senior leadership positions in the Antitrust Division of the U.S. Department of Justice. Most recently, she served as Deputy Assistant Attorney General for Civil and Criminal Operations. In that role, she supervised civil and criminal antitrust enforcement efforts and coordinated with antitrust and competition law enforcement agencies worldwide. During her tenure at the DOJ, Sonia oversaw a number of the Division’s most significant matters, including its challenges to the Anthem/Cigna and Aetna/Humana mergers. She also had responsibility for the Division’s international program, policy initiatives and competition advocacy efforts. She previously served as Deputy Assistant Attorney General for Civil Enforcement and as Chief of Staff and Senior Counsel to former Assistant Attorney General for Antitrust Bill Baer.
