Optimal outcomes in population health require both efficacious treatments and adherence to those treatments. Whether the treatment involves taking medication properly, making and keeping health care appointments, or self-managing other behaviours that influence the onset, course or prognosis of an illness; all other things being equal, success is determined by adherence behaviour. Patients, health care providers, researchers, funders and policy-makers, all have an interest in ensuring that effective biomedical and behavioural therapies for chronic illnesses are "used as prescribed". However, empirical studies have consistently found that levels of compliance or adherence are often far from optimal (1,2). Because the burden of illness in the population has shifted toward chronic diseases, the problem of poor adherence is of major concern to all stakeholders in the health care system. This is because the risk of poor adherence increases with the duration and complexity of treatment regimens and both long duration and complex treatment are inherent to chronic illnesses.

Across diseases, adherence is the single most important modifiable factor that compromises treatment outcome. The best treatment can be rendered ineffective by poor adherence. Our perspective is that an understanding of basic behavioural principles and models of behavioural change is relevant to adherence to treatment for all chronic medical conditions, and more helpful than a disease-specific approach to the issue.

Behavioural science offers useful theories, models and strategies that support best-practice approaches to delivering treatment. The effectiveness of adherence interventions based on behavioural principles has been demonstrated in many therapeutic areas. Examples include hypertension (3), headache (4), AIDS (5), cancer (6), heart transplantation (7,8), chronic asthma (9,10), diabetes (11), high cholesterol (12), obesity (13) and sun-protection behaviours (14) among others. Recent research has also evaluated interventions aimed at maintaining adherence to treatments targeting substance abuse in pregnancy (15); alcohol abuse (16); opioid addictions and methadone maintenance (17,18); substance dependence (19); cocaine abuse (20), and tobacco smoking (21).

Decades of behavioural research and practice have yielded proven strategies for changing people's behaviour. Such strategies can be used to help patients with diverse medical conditions (22,23), and can also be effective in changing the behaviour of health care providers (24) and health care systems (25).

Epidemiological research concerning the prevalence and correlates of poor adherence to treatment, and research on adherence to treatment for specific diseases is presented in the main text of this report. In this annex, the following are discussed from a behavioural perspective:

- the nature of poor adherence;
- a practical approach to conceptualizing and defining adherence;
- models to help explain determinants of adherence; and
- guidelines for assessment and intervention in clinical practice.