I work at Robert Mondavi Winery, and when guests visit, we like to ask them what percentage of wine made in California comes from Napa Valley. Most responses range from 25-75%. In fact only 4% of California comes from the Napa Valley, a deceptively small AVA (American Viticultural Area) that has 1/6 the planted acreage of Bordeaux, France. However close to 25% of the economic impact of the California wine industry comes directly from Napa Valley. The focus then is premium quality over quantity.
Wine came to California through two main channels - Spanish missions came first, bringing Catholicism and viticulture to California in the mid-to-late-1700s. The drive behind a true wine industry came decades later, in the Gold Rush of the 1840s. Hundreds of thousands of immigrants poured into the port of San Francisco, many of them looking to strike it rich in the California hills. Those coming from the wine cultures of western Europe brought their thirst for the vine with them. Before long we see the first commercial winery founded in Napa Valley - Charles Krug in 1861.
Napa Valley first made a name for itself in the latter half of the 19th century, becoming famous nationwide and having the first glimmers of success in the international market. Prohibition (1920-1933) and the Great Depression (1929-1930s) put an abrupt halt to all of that. In 1919 there were 140 bonded wineries in Napa Valley, in 1933 were were less than 100 in the entire United States. Louis Martini opened his eponymous winery in 1933, launch the rebirth of the Napa Valley wine industry. The next 30 years saw a lot of replanting and relearning what was lost over the previous 14.
I'm obviously biased, but I usually equate the start of the modern era of Napa Valley with the opening of Robert Mondavi Winery in 1966. The Mondavi family had owned and operated the historic Charles Krug winery since 1943, but Robert became increasingly convinced that Napa could compete with the best wines of the world and was driven to start his own, independent, winery to pursue that goal. Importantly, he also wanted to inspire and teach his contemporaries how to pursue excellence as well. A rising tide floats all boats. We see this influence in Chapellet for instance, who crushed their first vintage at Robert Mondavi Winery in 1968 while waiting for their construction to end up on Prichard Hill. The next pivotal moment is the famous Judgement of Paris in 1976 - Stag's Leap Wine Cellars and Chateau Montelena winning a blind tasting over several Grand Cru Burgundies and Classified Growth Bordeaux (Warren Winiarski and Mike Grgich, the respective winemakers, both were winemakers at Robert Mondavi Winery before reaching their fame in the Judgement of Paris.) The boost of the reputation of the whole valley from the Judgement and the constant national and internation marketing of Mr. Mondavi led the valley on its path to the luxury wine that it lives in today.
Currently 500+ bonded wineries exist throughout 16 AVAs that span 30 miles north to south. Napa is one of the very few places in the Northern Hemisphere that actually gets colder as you head south. This is because Carneros, the southern-most AVA of the valley, opens to the San Pablo Bay which brings cold maritime winds and fog. The hotter northern stretches of the valley create a pressure differential that sucks this cold weather up-valley, resulting in a nighttime temperatures that can be up to 50 degrees colder than the daytime highs. This diurnal shift is key for slowing down grape maturation and preserving acidity in the end wine. The unique weather pattern also is a large part of why subdividing the valley into 16 sub-AVAs is necessary. Calistoga at the northern end can be 20°F (or more) hotter than Carneros. While Zinfandel thrives in the north, Pinot Noir and Chardonnay are much more common in the south. The heart of the valley is home to the most famous (and expensive) examples of Cabernet Sauvignon with the Oakville and Rutherford AVAs being particularly distinctive. The "mountain AVAs" surround the valley, dividing the Mayacamas Mountains to the West and the Vacas to the east. Vineyards planted throught these regions are above the maritime fog layer and tend to show darker, deeper fruit as a result. Soil types vary widely throughout the valley as well, a result of active volcanic activity and the proximity to large bodies of water.
When it comes to actually drinking the wine, I admittedly have a strong preference for old world wines - and old wines! But there is something so satisfying and delicious in a well aged Napa Cab. I find they retain a core of pure, cherry-like fruit even into their 4th and 5th decades that I rarely, if ever, find in similarly aged Bordeaux. And that's really what got me, and keeps me, excited about working in Napa Valley and sharing the wines with my guests.