Dennis Tueller is perhaps best known for researching and developing groundbreaking training protocol, as exemplified in his published seminal article, "How Close is Too Close." In that text Dennis identified the critical concepts dubbed "proximics", which influenced a revolution in tactical training doctrine, coming to be known as the "Tueller Principle."

Beyond that noteworthy accomplishment, Dennis is internationally recognized and respected as a law enforcement instructor, acknowledged by peers in books and publications. He has served as adjunct instructor/coach for world-renowned training organizations including Thunder Ranch, Inc., Gunsite Training Center, U.S. D.O.E. Safeguards & Security Central Training Academy, Defense Training International, The Police Training Division, International Association of Law Enforcement Firearms Instructors and he is regularly invited to present at major law enforcement training conferences throughout the United States, Canada, and Europe. He currently teaches for Glock in their police firearms instructor and armorer training division.

In his career, Dennis has developed and presented hundreds of classes, lectures, live-fire range and shoot-house training courses, role-play exercises, personal coaching sessions, seminars, and video productions on practical shooting techniques, instructor development, patrol tactics and officer survival procedures, firearms safety, combat marksmanship and gun-handling, judicious use of lethal force, performance under stress, and winning mind-set. During his career with the Salt Lake City Police Department he received the Chief's Award for outstanding performance in developing firearms policy and providing firearms training. Dennis retired from the SLCPD after 25 years of service, at the rank of Lieutenant.