Over half of the world's population lives in cities. Cities are responsible for more than 60% of greenhouse gas emissions. WHO identifies several factors that negatively impact upon the health of urban populations: poor housing and transport conditions, pollution, waste management and inadequate sanitation all contribute to an increase in non-communicable and infectious diseases, as well as an increased risk of violence and injuries, particularly road traffic injuries. And health inequalities are particularly marked in urban areas. To address these challenges, WHO advocates a "systems" approach to urban health, i.e. taking into account all the determinants of health: the characteristics of urbanization are clearly identified as a priority determinant of health and well-being. To do this, health must be taken into account in all city policies.