It is important to understand what are the key principles that lead to livability and subsequently sustainability. Urban density is a number of people that live in each specific urban area.  It is believed that cities operate more efficiently when residents live in denser urban surroundings. Cities that have high densities tend to be more walkable and have greater transportation options and when successful, always implement mixed use regulations that create the type of urban fabric that is built for human users rather than prioritizing cars. This high density, mixed use approach to is leading the way towards sustainability by offer greener, less wasteful and high energy efficient way of life. For a city to be able to provide efficient transport it must have high demand, this demand, of course, comes from high density. In order to maximize public transportation and encourage walking and bicycling culture, the city must reduce travel time between live/work/shop/play. Dense cities hold creative environments of innovation and productivity (see The Space). Also, a compact city begs for great gathering places that serve as community nodes where people gather to connect. 

Urban density when not handled with care can be catastrophic not only for the cities inhabitants but for the environment. However, growing urbanization needs cannot be ignored and allowed to spiral out of control. Since WW2 growing cities have been struggling with their rate of growth desperately trying to find solutions to their housing crisis. One specific typology increases units without the significant reduction in quality of life seen in sprawls.  low-rise, high-density, a community centric approach with an emphasis on “livable scale” or "human scale". (see "Mixed Family Dwellings.") Density is essential to achieve sustainable urbanism. the compact city ensures a certain proximity to home-work-services in a mixed-use approach to urban planning which lends itself for walkability. Edward McMahon from the Urban Land Institute says that density is better achieved "without high-rises."

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Well planned density gives cities the opportunity for improved infrastructure (see green infrastructure) leading to cleaner and more energy efficient environment. A more concentrated density also lessens the possibility of human invasion of natural habitats. For a city to be livable however, population and functions must be dispersed at lower densities.” (Dessì , V. (n.d.) (2015)). This interplay between livability and density reduces per capita environmental demands while making infrastructure, and economic development much more effective, more cost-effective, and better connected!