Many brands of bitters began as medicinal tonics and soon found a home in cocktails as concentrated flavor stimulants that add a nice kick to the mix even though they are only used by the dash. The often secret formulas include a variety of herbs, fruits, spices, and roots distilled in a base liquor. In today's bar, bitters are essential, continue to evolve, and each brings its own qualities to each cocktail, so don't be afraid to experiment.