Do neighborhoods work like habitat for different kinds of households? City populations have rebounded in the past two decades as people who like city habitats have grown in numbers and in their share of the population. Mostly these are the Millennials - adults roughly 20 to 34-years-old, also known as Generation Y or the Echo Boom - who have delayed childbearing, marriage, and even household formation because of a combination of changing culture and economic necessity. Urban living makes sense for these young people. This article explores one of the most interesting questions for cities in the next 10 to 20 years: How many Millennials will stay there?