Throughout history, there have been periods when populations have tended to congregate together in towns and cities. The reasons for town development are numerous; in the early empires of the Middle East, water supply was a prime factor and in the city states of the Mediterranean lands, safety from raiders and pirates dictated collective defence. Sources of fresh water also played a part in the establishment of nucleated settlements in the lowlands of Great Britain, and the advantages of safety in numbers led to the growth of the medieval walled towns. With increases in populations in more peaceful times, people were encouraged to spread out to make their homes where they could grow more food, and, often spurred on by religious persecution, many travelled across the world to establish themselves in new lands. The Industrial Revolution and the growth of manufacturing industries brought people together again. The establishment of factories meant that livelihoods became dependent on employment rather than on subsistence farming through self-endeavour. This process of urbanization, the congregation of people together to live in towns, escalated through the twentieth century. In the older developed countries, like the UK, the well-established towns and cities have continued to expand; all the benefits of a high standard of living are much more economically provided in a centralised community. In the developing countries, the cities are an attraction to expanding rural populations seeking factory employment; throughout the world, urban centres are growing and, in some countries, the expansions are not planned or controlled. Thus, with much of the world’s population living in urban environments, the effect of such developments on elements of the hydrological cycle assumes a significant importance.