This review describes the mechanisms of lahars and debris flows, and their impacts on buildings, infrastructure and other valuables. Lahars are a type of debris flow restricted to volcanic debris. Both water-rich mass flows, having solid concentration from 40 to 80 % volume and density from 1,300 to 2,400 kg/m3, exhibit velocities ranging between 3 and 30 m/s, peak discharge as high as 48,000 m3/s, volume as large as 100 million m3, and can flow down valley as far as 100 km from the source. The review presents the array of methods and tools used in the field-based and experimental study of hydraulic, physical and rheological characteristics of volcanic and non-volcanic debris flows worldwide. These mass flows share several characteristics and effects with floods, but fewer studies have analyzed the wide range of impacts produced by lahars and debris flows. Damages are induced by three principal forces driving impacts: (1) hydrodynamic pressure, (2) hydrostatic pressure, and (3) the collisional forces of boulders acting as missiles. The impacts of debris flows are diverse, cascading and interacting because the size, the velocity and the proportion of rock debris mixed with water constantly change down valley. The review will complement previous studies in three ways: 1) we investigate how debris-flow characteristics and processes lead to damage mechanisms on buildings, infrastructure and lifelines; 2) we explore the broad range of valuable assets impacted by debris flows, and; 3) we analyze the physical vulnerability of valuables, leading to a review of indicators, matrices and fragility functions, which may lead to better understand loss. The review shows that economic impacts of lahars and debris flows extend well beyond the immediate costs of loss of life and asset damage. Enhanced understanding of flows and their impacts can improve risk mitigation and land use planning.