Brown sugar is a stronger flavored, more moist sugar often used in baking and glazes. When a recipe calls for brown sugar and you don't have it on hand, try a quick substitution.
Make Your Own
Brown sugar is really just cane sugar flavored with molasses (which is itself a by-product of sugar). Make your own by adding 1 tsp. (lighter) to 2 tsp. (darker) of molasses to a cup of sugar, and use cup-for-cup.
Use White Sugar
Using white sugar in place of brown will result in a different, lighter flavor and may alter the texture in baking, but it will work in a pinch.
It's not recommended for baking, but if you're looking for a substitute in a sauce, marinade, or rub, molasses alone both sweetens and adds a distinctive flavor. Add gradually, to taste, and add white sugar if needed.
If you don't have molasses but have plenty of honey, it can be used in place of brown sugar, but expect a different result. In baking, use just under 1 cup of honey per cup of brown sugar, and decrease the amount of liquid in the recipe by 3 tbsp.
Brown Sugar Substitute
Less improvised, but a substitution all the same, commercial brown sugar substitute (such as Splenda Brown Sugar Blend) can be used in place of regular brown sugar to cut calories. Because sucralose is much sweeter than sugar, replace 1/2 cup of brown sugar substitute for 1 cup of regular brown sugar.