The defining characteristic of Manhattan's Upper East Side , a two-square-mile grid scored with the great avenues of Madison, Park and Lexington , is wealth. While other neighborhoods are affected by incursions of immigrant groups, artistic trends, and the like, this remains primarily an enclave of the well-off, with tony shops, clean and relatively safe streets, well-preserved buildings and landmarks. It also has some of the city's finest museums , all in a compact strip running on or near Fifth Avenue, known as Museum Mile: the Frick Collection and the Guggenheim Museum are the best (and best known), but the Whitney Museum of American Art , the Jewish Museum and the Museum of the City of New York each command attention.