McDermott Will & Schulte, a global law firm

Stéphane Dionnet is co-head of McDermott’s Antitrust Mergers Focus Group and has over 20 years of experience practicing international competition law, and has experience liaising with competition authorities in the European Union, United States and Asia. Stéphane has advised global corporations on how to navigate complex multijurisdictional merger control matters. He has represented clients in numerous cartel investigations and has also successfully assisted companies in obtaining conditional immunity from the European Commission and other competition law agencies in and outside the European Union.

Stéphane has experience across a wide range of sectors including consumer products, energy, entertainment, financial services, technology, pharmaceuticals, retail, e-commerce and telecommunications.

Stéphane is recognized by Lexology Index as a Recommended Lawyer in Competition (2025) and is listed in Global Competition Review’s GCR 100 (2025). He is also recommended by The Legal 500 EMEA for Competition for 2023–2024. Clients describe Stéphane as “an excellent competition lawyer with a great strategic sensibility” and praise him as being “great to work with,” noting his “fantastic work product.”

Earlier in his career, Stéphane worked for three years within the “Merger Network” of the European Commission’s Directorate-General for Competition. He was case-handler in high-profile transactions, including phases 1 and 2 with remedies.

Ryan Tisch is the global head of McDermott’s Antitrust Group. He provides strategic antitrust counsel for boards of directors, chief executive officers, and key business stakeholders as they maneuver government regulations around the world.

Ryan is known for securing approvals for significant mergers facing review in multiple jurisdictions, including some of the largest mergers in the communications, manufacturing, technology, and food and beverage industries. As the digital economy exposes traditional businesses to new competition, clients also turn to Ryan to advise on their evolving business platforms and go-to-market strategies.

Throughout his global practice, Ryan handles matters extensively before the United States Department of Justice and the Federal Trade Commission and manages substantive merger review matters before the United Kingdom’s Competition and Markets Authority, the Competition Tribunal of South Africa, the Australian Competition and Consumer Commission, and China’s State Administration for Market Regulation, among other international authorities. Ryan has a proven track record of leading merger matters from the auction process to a successful resolution, including through trial.

In addition to his deal work, Ryan helps clients mitigate risk on a day-to-day basis. He advises on complex operational issues such as joint venture agreements, trade association activities, standard setting and data exchange. Ryan has particular experience navigating clients’ vertical relationships and distribution systems, and managing antitrust issues associated with resale price maintenance, price discrimination and loyalty incentives.