The martini is a cocktail traditionally made with gin and dry white vermouth, though in recent years substituting vodka for gin has become more popular than the original recipe. Over the years, the martini has become perhaps the most well-known mixed alcoholic beverage. H. L. Mencken once called the martini "the only American invention as perfect as the sonnet" and E. B. White called it "the elixir of quietude." It is also the proverbial drink of the one-time "three-martini lunch" of business executives, now largely abandoned as part of companies' "fitness for duty" programs.

While variations are many, a standard modern martini is a 5 to 1 ratio, made by combining approximately two and a half ounces of gin and one half ounce of dry vermouth with ice. Many Europeans, however, prefer somewhat less vermouth — about a 6:1 proportion of gin/vodka to vermouth. Because vermouth mixes easily and uniformly with its solvent (gin or vodka), a martini should always be mixed in a stirring glass. Many bartending schools insist that a beverage shaker tends to dull the taste of the vermouth, and some argue that it sharpens the taste of gin by "bruising" the liquid. However, it is relatively common to see a bartender mix a martini with a shaker due in part to the influence of superspy James Bond, who asked for his martinis "shaken, not stirred." This may be attributed to the fact that Bond drank vodka martinis and not gin martinis. The ingredients are mixed then strained and served "straight up" (without ice) in a chilled cocktail glass, and garnished with either an olive or a twist (a strip of lemon peel, usually squeezed or twisted to express volatile citric oils onto the surface of the drink).

While the standard martini may call for a 5:1 ratio of distilled spirits to vermouth, many aficionados may reduce the proportion of vermouth drastically. This gave rise to stories such as martinis being made by just passing the cork of the vermouth bottle above the glass, along with similar conceits about how little vermouth, i.e., how "dry," one's martinis are (see history below).

Another common but controversial variation is the vodka martini, which is prepared in exactly the same way as a standard martini, with vodka being substituted for gin as the base spirit. In the 1990s, the vodka martini supplanted the traditional gin-based martini in popularity. Today, when bar and restaurant customers order "a martini," they frequently have in mind a drink made with vodka. Martini purists decry this development: while few object to the drink itself, they strenuously object to it being called "a martini." The martini, they insist, is a gin-based cocktail; this variation should be designated as such, with the name "vodka martini" (it may also be called a "vodkatini" or a "kangaroo"). Further confusion may arise from confusing Martini vermouth, a brand of vermouth, with the martini cocktail.

A more recent development that further offends martini purists is the use of "martini" (or the suffix "-tini") to refer to any flavored vodka cocktail served straight up in a cocktail glass. For example, the appletini, the chocolatini, or pineapple martini.