The main ingredient in pesto sauce is basil. Traditionally, pesto is made from fresh basil leaves, which give it a distinct aroma and flavor. Other key elements in the classic pesto recipe include pine nuts, garlic, Parmesan cheese, and olive oil. The ingredients are typically blended together to create a thick, fragrant paste that can be used in a variety of dishes. While basil is the star ingredient, variations of pesto may incorporate other herbs, nuts, or even different cheeses, allowing for creativity in preparing this versatile condiment.