Rue (Ruta graveolens) is a bitter, pungent Mediterranean herb with a sharp, citrus-and-pepper-like flavor, historically used in small amounts in some Greek, Italian, and Middle Eastern cooking. It's a minor, uncommon ingredient in modern kitchens, largely because it's toxic in more than very small doses and can cause skin sensitivity to sunlight -- most recipes that call for it use only a leaf or two, and it shouldn't be treated as an everyday herb. Where it is used, it's typically paired with garlic, lemon, and olive oil, with its bitterness balanced by acidic or sweet ingredients.