A typical single shot of espresso (about 1 fluid ounce, or 30 ml) contains roughly 65 milligrams of caffeine. By comparison, an 8-ounce (240 ml) cup of drip coffee has about 95 mg, so espresso is more concentrated per ounce but a single shot usually delivers less total caffeine than a regular cup. Actual amounts vary with the beans, roast, and how the shot is pulled.
Source: Wikipedia, Espresso