Porter is a style of beer in the ale family which has a dark colour. It was developed in the 18th century, originally from the use of highly dried brown malt, a roast malt aroma and hop bitterness. It is generally brewed with soft rather than hard water. Strong Porter was called "Stout Porter", which was later shortened to just "Stout". Guinness Extra Stout was originally called "Extra Superior Porter" and was only given the name Extra Stout in 1840.

Stout grew into its own recognised style. but there is still much debate today on whether this division is appropriate. Usually the deciding factor in whether a particular ale is a porter or a stout is strength. After the invention of malted barley roasted until black, also known as patent malt, in 1817, to impart a darker colour, which also gives a distinct burnt taste to the beer, Irish brewers dropped the use of brown malt, using patent malt and pale malt only, while English brewers continued using some brown malt, giving a difference in style between English and Irish porters and stouts. Stouts sometimes also use roast barley, unmalted barley roasted black, that can impart a flavour of coffee.

The microbrew revival of the past twenty years has led to somewhat of a resurgence in the popularity of the style, with many new varieties available around the world. The style is particularly prominent and popular in the United States.