Abstract The desert can provide a harsh environment for engineering construction. Potential hazards are discussed in earlier Chapters, particularly Chapters 2 and 3 , and include extreme temperature and humidity fluctuations, aggressive ground and groundwater conditions, wind-blown sand and dust, and flash floods. The major influence that water has on the behaviour of the ground and the performance of engineering structures in hot deserts is often not appreciated or is underestimated. Rainstorms, although infrequent, can be of high intensity. It may not rain for months and then the annual average may fall in 1 day, leading to flows in normally dry wadis, flash floods and the filling of ephemeral lakes. In addition to water from rainstorms and runoff, construction and urbanization may introduce infiltration of water into soils that are normally dry. This can lead to changes in strength or volume that would not have occurred naturally. The response of normally dry soils to wetting, and the identification and assessment of ‘collapsible’ and ‘expansive’ soils, is discussed in Chapter 8 . Approaches to mitigating the effect of volume changes associated with wetting on the performance of engineering structures are discussed in this Chapter (Sections 10.2.3, 10.2.4 & 10.6). Groundwater allows the movement of salts within the ground and, particularly where it occurs within a few metres of the ground surface, this can give rise to salt damage to structures and adversely affect roads and earthworks. The factors affecting salt accumulation, leading to aggressive ground and groundwater conditions, are described in