In the red hot world of craft beer, one style of beer is hotter than all the rest—the India Pale Ale. The style, which is known for its strong hop character, dates to the 1800s and its popularity has seen peaks and valleys before. But the modern day IPA is currently the king of craft beer. It is the fastest-growing and best-selling style of craft beer by nearly any metric. According to market research group IRI, which tracks off-premise beer sales, IPA sales were up 36 percent in the first half of 2013. Another market research firm, GuestMetrics, which measures on-premise sales, said the IPA style was the fastest-growing beer category in 2012, posting unit growth of 39 percent.



Harpoon IPA Source: Harpoon Brewery

Then there is the Great American Beer Festival, the world's largest beer competition. Since 2001, the most-entered category has been the American-Style IPA, which saw 252 entries in 2013.

But while IPA may reign supreme today, it wasn't always the case. Back in 1993, when Harpoon Brewery, then-based in Boston, released its Harpoon IPA as a summer seasonal, the style was little known. The recipe for Harpoon's IPA was based on a traditional English-style IPA, but was tweaked to use American hops and Harpoon's signature yeast strain, which provided a malt profile designed to balance out the hop bitterness. (Read more: When it's time to change: Craft brewers cash out) While there were a few IPAs being brewed in the Pacific Northwest, Harpoon's IPA was the first on the East Coast and with its hoppier and more bitter flavor, it was unlike anything else in the marketplace.

"Back then no one knew what this stuff was," said Harpoon co-founder Dan Kenary. "So there was a very steep learning curve for everybody and that was a real challenge."