California's Napa Valley is undoubtedly the most famous wine region in the United States. As you might expect from a California wine region, Napa exudes star quality. Even 25 years ago, the valley bustled with activity and attracted tourists from all over the country. Today Napa Valley is one of the world's flagship wine regions, with approximately 400 wineries and a reputation for top-quality wines. For many wine lovers, Napa Valley is synonymous with cabernet sauvignon. This late-ripening grape flourishes in Napa Valley's warm days and cool nights. While cabernet sauvignon grapes thrive all over the Napa Valley AVA, they do particularly well in the Rutherford and Oakville sub-regions, which are AVAs in themselves. (Napa Valley has 15 subregions, all of which are also AVAs.)