Following a strong tradition of community psychiatric epidemiological research since the 1980s, the Department continues to provide and disseminate a wealth of locally relevant data on the prevalence, distribution, correlates, treatment and outcome of mental disorders in Hong Kong. Our findings have had a widely-felt local impact, including the introduction of broad-based mental health advocacy and new government policy on mental health service and training. Our collaboration in the World Mental Health Surveys has contributed to the understanding of mental disorders by examining the prevalence, risk factors, disease burden, interventions and barriers to service use. From the data we have produced, we have disproved the myth that mental disorders such as depression and anxiety are uncommon in Chinese society. In our latest territory-wide face-to-face community survey, the Hong Kong Mental Morbidity Survey, we further demonstrated that while mental disorders are as common in Hong Kong as in developed Western countries (13% of the local population), the under-utilisation of mental health services andunder-treatment are highly prevalent. This is a matter of great urgency for public health. Moreover, contrary to the widespread perception that common mental disorders often resolve spontaneously, our longitudinal data indicate that in the absence of effective interventions they are persistent and can worsen with comorbid complications over time. In addition, our series of large scale community-based studies identified not only the epidemiology of common sleep disorders and problems, such as obstructive sleep apnoea, insomnia, nightmares and parasomnia across the lifespan, but also developmental issues in sleep disorders and their relationship with mental and physical problems.