Urban areas concentrate people from many different backgrounds, but these people are not distributed uniformly or randomly in space. This is the key issue that segregation studies want to raise, and it is one that has long had social and policy implications. In many cities, similar groups of people live close to each other, with few opportunities to share residential, work or leisure spaces with other groups — this in turn results in fragmentation and isolation. These unequal distributions are often a consequence of historically constructed social hierarchies, such as race and class systems, that allow certain groups to enjoy urban goods while others suffer the consequences of urban ills.