In this Special Issue introductory paper, we present the Tomorrow’s Cities Decision Support Environment (TCDSE). As the negative impacts of natural hazards continue to escalate around the world due to increasing populations, climate change, and rapid urbanisation (among other factors and processes), there is an urgent requirement to develop structured and operational approaches towards multi-hazard risk-informed decision making on urban planning and design. This is a particularly pressing issue for low-to-middle income countries, which are set to be impacted ever more disproportionately during future natural-hazard events if the “business as usual” urbandevelopment approach continues unabated. Urban poor residents of these countries will significantly suffer under risk-insensitive development trajectories. The proposed TCDSE addresses this crucial challenge. It facilitates a participatory, peoplecentred approach to risk-informed decision making, using state-of-the-art procedures for physics-based hazard and engineering impact modelling, integrating physical and social vulnerability in a unified framework, and expressing the consequences of future disasters across an array of stakeholder-weighted impact metrics that facilitate democratisation of the risk concept. The purpose of this introductory paper is to provide a detailed description of each component of the TCDSE, characterising related data inflows and outflows between modules. We conclude with a short operational end-to-end demonstration of the TCDSE, using the Tomorrowville virtual urban testbed. Individual components of the TCDSE are further dealt with in detail within subsequent papers of this Special Issue.