Floods occur from a combination of various hydrological, climatological, and land-use conditions. In this article, we review recent studies to identify primary causes of floods in different regions. These studies suggest that changes in precipitation intensity or amount are responsible for increasing floods in coastal cities, which may also be impacted by sea-level rise and an increase in the incidence of tropical storms due to global warming and land modifications at different scales enhance vulnerability to flood risks. They indicate the current challenges in flood research include: (i) the integration of data collected at various scales; (ii) the quantification of various feedbacks and uncertainty between climatic and hydrologic systems; (iii) the combination of climate and land-use models for flood assessment; and (iv) linking flood damage caused by changing climate or land use with spatially explicit socioeconomic data. Specifically, an integrated assessment method is needed to unravel the complex interactions among human systems, terrestrial systems, and climate systems that result in flood risks at multiple scales.