Terrorist attacks are becoming more deadly, and terrorist groups are beginning to engage in tactics with the specific aim to cause maximum casualties. Such attacks range from those targeting personnel, e.g. in crowded places such as Istanbul Airport and Manchester Arena, or those targeting infrastructure, e.g. Metrojet Flight 9268. In order to ensure that engineers provide effective and efficient protection systems, we must fully understand how a blast load develops in a complex, crowded environment, and how the presence of obstacles and obstructions alters the propagation of a blast wave. Whilst it is known that a blast wave will reflect off and diffract around an obstacle, accurate quantification of this effect and a detailed understanding of the mechanisms governing this behaviour have, to date, eluded researchers in the field of blast protection engineering. This project aims to address this knowledge gap through experimental work at a world-leading facility, involving direct measurement of blast wave parameters both at the source and downstream of the obstacle. This experimental work will be supplemented with cutting-edge numerical analysis, using tools specifically designed for simulation of blast wave propagation in complex environments. Both of these approaches will then be combined to develop generalised relationships for the interaction of blast waves with obstacles, enabling a semi-empirical tool to be developed for rapid calculation of the flow field surrounding an obstacle following detonation of a high explosive. This project aims to prove the concept of porous blast barriers, i.e. barriers comprising a series of smaller obstacles rather than a large, imposing, (typically concrete) monolithic structure. Such designs, it is envisaged, will become the next generation of urban blast protection strategies: engineered systems with tailored properties to achieved maximum blast protection, but compatible with a modern, open, green city. Blast protection systems that do not look like blast protection systems.