Professor Clark is an internationally recognised expert in heart failure, and is a frequently invited speaker to conferences of all sorts. He has published over 400 papers, principally in the field of heart failure, but including papers on primary care and even contraception. He is past chair of the British Society for Heart Failure, and past chair of the Heart Failure Alliance, which has led on advising the All-Party Parliamentary Group on Heart Failure in its production of its report on heart failure. He chairs steering committees for multicentre clinical trials, is on the National Audit for Heart Failure steering group, and is on the editorial boards of national and international medical journals. He is the senior editor of the Oxford Textbook of Heart Failure.