Debris flows and debris avalanches are among the most dangerous and destructive natural hazards that affect humans. They claim hundreds of lives and millions of dollars in property loss every year. The past two decades have produced much new scientific and engineering understanding of these occurrences and have led to new methods for mitigating the loss of life and property. These 17 papers pull together much of this recent research and present it in these categories: (1) process, (2) recognition, and (3) mitigation. Much of this work results from cooperative efforts between GSA's Engineering Geology Division and Quaternary Geology & Geomorphology Division.