This monograph, written by world-recognized experts, is the first book that reviews a variety of problems in different fluid mechanics disciplines that led to the concept of canopy, or penetrable roughness. Despite their diversity, many flows may be theoretically united by means of introducing distributed sinks and/or sources of momentum and heat and mass. Terrestrial vegetation, historically the first example of canopies, creates specific features of turbulence. Aquatic canopies exhibit a range of behaviour depending on the depth of submergence, geometrical forms of the obstacles and the patterns of their relative locations. These and other flows in engineering and environmental situations over surfaces with many obstacles are reviewed in terms of general concepts of fluid mechanics. They have been subject to examination by field-scale and laboratory experiments, and have been modelled and simulated using a variety of computational techniques. Distinct regions of the flows are identified. Application of the flow modelling is also relevant to predicting the dispersion of pollutants in these complex flows, particularly for releases in street canyons and fire propagation.