Dennis graduated from University of Cardiff in 1998 and undertook his junior doctor training in South Wales, during which he obtained his degree in Doctor of Medicine (MD) studying the impact of respiratory viral infections in Cystic Fibrosis. He then completed his Specialist Registrar in Chest Medicine and General Internal Medicine in the Eastern Deanery. He was appointed as Consultant Respiratory Physician in Cambridge tertiary referral Centre for Lung Infection at Papworth Hospital, specialising in patients with Cystic Fibrosis and non-CF bronchiectasis in 2009. In 2012, he moved to the Liverpool Heart and Chest Hospital and assisted in the development of the Knowsley Community Respiratory Service (KCRS) that provides respiratory support to over 8000 patients with COPD, asthma and bronchiectasis. He is also a Consultant of the Adult Cystic Fibrosis Unit, the Liverpool and Knowsley home oxygen service and the Regional TB service. He is the Chair of the Drugs and Therapeutics committee at Liverpool Heart and Chest Hospital. He is an Honorary Senior Clinical Lecturer in University of Liverpool and also an examiner for The Royal College of Physicians. He is the Chief Investigator of a multicentre clinical study in COPD and also a Principal Investigator of various international clinical trials in COPD, bronchiectasis and non-tuberculous mycobacterial diseases. Dennis has published articles in numerous COPD, Bronchiectasis and Cystic Fibrosis peer-reviewed journals and has also been an editor of two books on Cystic Fibrosis.