Craft brewers have broken into the big leagues of brewing -- breaching 11 percent of beer produced in the U.S. by volume in 2014, marking the first time that small and medium-sized breweries have topped double-digit production levels in a market long dominated by a handful of large brewers. Craft beer's share last year was up sharply from 2013's 7.8 percent.

The one-year growth is striking -- craft brewers produced 18 percent more beer last year than the year before. Meanwhile, sales of craft beer rose 22 percent to $19.6 billion, according to numbers released this week by the Brewers Association, an organization of craft brewers

“This steady growth shows that craft brewing is part of a profound shift in American beer culture -- a shift that will help craft brewers achieve their ambitious goal of 20 percent market share by 2020,” Bart Watson, chief economist at the Brewers Association, said in a statement. 

Part of the reason the nation is drinking more craft beer is rooted in the fact that millennials are driving sales. Craft beer consumption is highest among people ages 25 to 34, a majority of whom tell pollsters that the beer they drink is a reflection of their identity, according to market research by Mintel

“The appreciation of the craft nature of the category is evident in the fact that drinkers of craft offerings are significantly less likely than drinkers of regular domestic beer to say the purpose of drinking is to get drunk,” Beth Bloom, a food and drink analyst for Mintel, said in a statement

The specialty organization’s definition of a craft brewer has shifted over the years. Today, a craft brewer is defined as an outlet that produces 6 million barrels of beer or fewer per year. The owners must be independent and use traditional methods of brewing.

That upper limit was raised from 2 million barrels a year in 2010, possibly as a way to help the brewers' group attain its goal of capturing 20 percent of market share by volume by 2020. The new limit is high enough to allow the group to include medium-sized brands such as Yuengling, which produces 3 million barrels of beer a year, CNBC reported

Even as production and sales grow, the nation's 3,418 craft breweries face challenges. Craft beer, traditionally sold at a premium compared with other mass produced beers, accounted for roughly 19 percent of sales in the $101.5 billion U.S. market in 2014.  

Just four major companies -- Anheuser-Busch, Heineken, SABMiller and Carlsberg make up 70 percent of beer sales worldwide, according to CNN. Anheuser-Busch, which produces Budweiser, Bud Light and Corona (America’s top imported beer), garners about 30 percent of total sales on its own, according to Statista.