There is increasing recognition of the value of formative evaluation, which aims to influence programme development constructively from an early stage by providing ongoing evaluation input to the programme planning and decision making process. Formative evaluation has been applied by the authors as the first phase of their evaluation of the Heartbeat New Zealand programme, a heart disease prevention programme which utilizes health promotion strategies. The role of formative evaluation in development and improvement of Heartbeat New Zealand is illustrated using four examples of formative evaluation activities. These are the use of formal reports and recommendations, interviews with programme personnel, literature review, and pilot project participant questionnaires. Strategies used by the authors to influence programme operation based upon constructively critical examination, while seeking to maintain the cooperation and support of key programme participants are discussed.