Roux ("roo") is used to thicken sauces and soups. Pre-cooking flour allows the starch granules to swell and absorb moisture, and lets you thicken a sauce base without the flour clumping or forming lumps. Rouxs are also used to deepen the flavor of a sauce: browning the flour gives it a nutty, toasted flavor.
Colors of roux
Roux can be white, blond, brown, or dark. The color just depends on how long you cook the fat-flour mixture. While none are better or worse than the others, it does affect flavor and how much the roux can thicken. For an in-depth description and photos of different types of roux, see our All About Roux article.
Cook and stir
To make a basic roux, use equal weights of fat and flour. Four ounces of fat and four ounces of flour equal about 8 ounces of roux (moisture will evaporate). If you don't own a kitchen scale, one tablespoon of flour equals about ¼ ounce. One tablespoon butter = ½ ounce. Butter is the most commonly used form of fat; other fats can be used, but will have a different flavor. Melt the butter over medium heat; slowly add the flour to the butter, whisking constantly. Within 2 to 3 minutes the roux will have a consistency of a cake frosting. A white roux is done when the flour loses its "raw" smell and begins to develop a toasty aroma. Darker roux are cooked, stirring constantly, until the desired color. If you're not adding liquid, immediately remove the pan from the heat and transfer the roux to another container to cool. Be very careful: the hot fat-flour mixture can cause painful burns. Refrigerated or frozen roux will keep well for up to two months and can be added directly to soups or sauces for quick thickening.
For step-by-step instructions on making roux, see our photo tutorials: