After World War II, suburban job and population growth in the United States far outstripped that of cities, leading many to worry that downtowns were doomed. In recent years, however, many cities have revived their fortunes by fashioning downtowns that are attractive and — for the first time in decades — drawing in new residents. Once-forlorn urban centers from San Diego to Philadelphia are now busy construction zones that are filling up with trendy shops and restaurants. But despite the good news, downtowns are still grabbing only a tiny fraction of metropolitan growth. Some skeptics worry that the downtown renaissance is fragile, largely built on upscale shopping and entertainment — relatively new trends that could easily change. But others believe downtowns, having once again become the most vital parts of many cities, will provide a model for future development — even in the suburbs.