The Aztecs invented it.

Seriously, that's it. When the Spaniards encountered the Aztec empire back in the 1500s, the locals were making a sauce called ahuaca-mulli, which means "avocado-mixture." The dish was prepared by mashing avocados, sometimes with tomatoes and onions. Sound familiar? Add a few hot peppers and a touch of cilantro, and you've got modern guac.

As the Spanish tended to do with all good ideas from the New World, they stole the best ideas from the natives and brought what they could back to Spain. Unfortunately for our friends in Europe, avocados don't grow particularly well in the Old World, making fresh guac the province of the Americas for the next several hundred years. Sailors and travelers ate various forms of the stuff, notably the English who made avocado paste, called it "midshipman's butter," and spread it on their hardtack.

The word "avocado" descends linguistically from the ancient Nahuatl (a dialect of ancient Aztec) word ahuacatl, meaning "testicles." Some language experts think that the conquistadors combined the Nahuatl with the Spanish "abogado" (lawyer) to make the present word for our favorite green fruit. So, when you're eating guac, you're chowing on mashed lawyer balls.

Most American avocados come from California, where there over 6,000 avocado groves dot the southern part of the state. Most of those avocados are of the Hass variety (Haas is a common misspelling); Florida avocados are larger, more fibrous, and taste worse, like everything from Florida. Outside the US, avocados are grown in Mexico, Peru, Chile, Argentina, Spain, Israeal, South Africa, Australia, and New Zealand. Current US policy essentially forbids the importation of avocados from Mexico, driving up domestic prices and making it nearly impossible to get a decent avocado outside of California for less than two bucks.

Modern guacamole comes in many forms. In some taquerias, it's large chunks of barely-cut avocado, in others it's a paste with a lively mixture of tomatoes, onions, and cilantro. And in some unfortunate cases, it's a soupy liquid with equal parts avocado, water, and sour cream. At any rate, it's very difficult to improve on the recipe that the Aztecs pioneered hundreds of years ago.The recipe below would probably bring a smile to Montezuma himself.

Guacamole Recipe

This is the recipe I use, cribbed from God knows where. Hope you like it.

Two Hass avocados. None of that Florida junk.
A tomato (vine-ripened, please. No supermarket gas-inspired flavorless crud.)
Half of a small onion
Some cilantro
A serrano pepper
Salt
Pepper
Lime juice

Dice the tomato and the onion. Mash the avocado, and squeeze some lime juice on it (this prevents it from turning brown). Mix it together; if the ratio of avocado to onion/tomato looks off, add more of whatever you like. Dice the pepper and add enough to give it just a touch of heat. Salt, pepper, and cilantro to taste. Put it on anything.

Sources / Further Reading

The California Avocado Commission

The Gourmet Sleuth's Guacamole page

Wikipedia

America's First Cuisines by Sophie D. Coe.