The degree to which a city is sustainable is affected both by the form of the urban street block and also by the composition of the activities it accommodates. The way in which the street blocks are designed and the land use mix within street blocks also affects the quality of the built environment. Current conventional wisdom adopted by those in the field of sustainable development rejects the cruder notions of land use zoning, in favour of subtler urban structuring, based upon a mix of uses and activities. The traditional city with residential and office accommodation arranged over ground floor shopping streets is often cited as a model arrangement for a lifestyle which is not dependent upon high levels of mobility. It is also argued that a city with a fine grain of land use, rather than the homogeneous zones of residential commercial or industrial uses, common in modern metropolitan areas, is more likely to reduce the need for travel, and, incidentally, also be more likely to create an interesting and liveable environment. There is little doubt that a city is judged by the quality of its public streets and squares: by their form, the façades which enclose them, the floor plane on which visitors tread, and the great sculptures and fountains which delight the eye. It is, however, the size, function and structure of the street block which gives form to public space and contributes to the vitality of those spaces. This chapter examines the various ideas about the form and function of the street block and its role in structuring the city, analysing, in particular, the street block in a sustainable city.