Dennis has a general litigation practice and has handled a wide range of civil cases, including antitrust, trademark and RICO cases; commercial disputes; insurance coverage litigation; and product liability claims. He has accepted court appointments in criminal cases, including murder cases. Dennis is a Fellow of the American College of Trial Lawyers (and Regent, 2001 - ), and a Fellow of the International Academy of Trial Lawyers (and member of the Board of Directors, 2000 - ). He is coauthor of The Deposition Handbook (4th Ed.) and of The Expert Witness: Law and Practice, and is listed in The Best Lawyers in America.

Dennis R. Suplee is on the Advisory Board for The Sedona Conference.