A wide variety of climatic factors influence the occurrence of debris flows, which include mudflows and lahars (see Chapter 2). Climatic factors are an important subject for a better understanding of hydrologic response of soils and of how global climate change can influence debris-flow activity. In addition, climatic factors are essential for developing debris-flow warning systems. Climatic factors have extreme spatial and temporal variability. Rapid infiltration of prolonged intense rainfall, causing soil saturation and a temporary increase in pore-water pressure, is generally believed to be the mechanism by which most shallow landslides, and more specifically debris flows, are generated during rainstorms (Iverson, 2000). This chapter focuses on the primary and secondary climatic factors that influence the occurrence of debris flows. Primary climatic influences are those that directly trigger debris flows, such as intense rainstorms or rapid snowmelt. Secondary climatic influences are those, such as antecedent rainfall or antecedent snowmelt, that influence whether debris flows are triggered during an earthquake, volcanic event, or intense rainstorm. Thresholds for the triggering of debris flows will also be discussed, including application of rainfall thresholds for hazards assessment and mitigation such as warning systems. In addition to short-term climatic influences, the possible effects of longer term climatic changes are examined. This chapter does not attempt to present new scientific material on this subject, but does try to summarize the extent of worldwide research to gain a better understanding of this complex subject.