An intensely flavored, big, dark ale. Roasty, fruity, and bittersweet, with a noticeable alcohol presence. Dark fruit flavors meld with roasty, burnt, or almost tar-like sensations. Like a black barleywine with every dimension of flavor coming into play.
Variations exist, with English and American interpretations (predictably, the American versions have more bitterness, roasted character, and finishing hops, while the English varieties reflect a more complex specialty malt character and a more forward ester profile). The wide range of allowable characteristics allow for maximum brewer creativity.
Three Floyd's Dark Lord, Bell's Expedition Stout, North Coast Old Rasputin Imperial Stout, Stone Imperial Stout, Samuel Smith Imperial Stout, Scotch Irish Tsarina Katarina Imperial Stout, Thirsty Dog Siberian Night, Deschutes The Abyss, Great Divide Yeti, Southampton Russian Imperial Stout, Rogue Imperial Stout, Bear Republic Big Bear Black Stout, Great Lakes Blackout Stout, Avery The Czar, Founders Imperial Stout, Victory Storm King, Brooklyn Black Chocolate Stout
You can view the complete style details on the BJCP website. The 2008 style guide is obsolete but there is a similar 2015 style that you can view below...