Dengue is one of the most important emerging arboviral diseases in the world. It has been estimated that 3.6 billion people in tropical areas, and approximately 120 million travellers, are at risk of dengue. More than 500,000 people with severe dengue require hospitalisation each year. A large proportion of these are children, and about 2.5 % of those affected die. The incidence of dengue has increased 30-fold over the last 50 years, and the geographic range of the virus and its vectors has also expanded. The disease is now endemic in tropical and subtropical regions in more than 128 countries. Dengue transmission involves a complex ecology, contributed through various factors such as virus, vector, host and environment. This chapter presents an overview of the current knowledge of dengue, its characteristics and ecology. It then discusses determinants of dengue and how climate change will impact its future distribution and transmission dynamics.