Cardiovascular diseases (CVD) are the leading cause of death and a significant source of health inequalities in New Zealand. Patients who have had an acute coronary syndrome (ACS) account for approximately half of all major CVD hospitalisations. For such patients, readily available, cheap and safe medications can more than halve the risk of a CVD event over a 5-year period. However, international evidence indicates that approximately half of patients are non-adherent to preventive medications. Information on the extent of the problem in New Zealand is limited. The proposed thesis aims to assess the extent of non-adherence to preventive medication post ACS in New Zealand, and investigate whether adherence differs between population subgroups and in different parts of the country. It also aims to determine whether differences in adherence predict difference in CVD outcomes, to estimate the impact of non-adherence on CVD event rates and to identify the stages of the prescribing-dispensing cycle where interventions are most likely to improve adherence.