Cities are increasingly expanding their boundaries and populations and as stated ’from the climatological point of view, human history is defined as the history of urbaniza tion’. Increased industrialization and urbanization of the re cent years have affected dramatically the number of the ur ban buildings with major effects on the energy consumption of this sector. It is expected that 700 million people will move to urban areas during the last decade of this century. The number of urban dwellers has risen from 600 million in 1890 to 2 billion in 1986 and if this growth continues, more than one - half of the world’s population will live in cities by the end of this century, where 100 years ago, only 14 per cent lived in cities and in 1950, less than 30 per cent of the world population was urban. Today, at least 170 cities sup port more than one million inhabitants each. As estimated, in the United States, 90 percent of the population is ex pected to be living in, or around, urban areas by the year 2000. Estimations show that urban populations will occupy 80 % of the total world population in 2100.