Place the basil leaves, pine nuts, and citrus juice into the bowl of a food processor and pulse several times to blend.
Add the garlic and Parmesan or Romano cheese and pulse several times more. Scrape down the sides of the food processor with a rubber spatula or wooden spoon to make sure it is all getting evenly blended.
While the food processor is running, slowly add the olive oil in a steady small stream. Adding the olive oil slowly, while the processor is running, will help it emulsify and will help to keep the olive oil from separating. Stop when needed to scrape down the sides of the food processor and make sure it is all blended evenly.
Stir in salt and freshly ground black pepper, add more to taste.
You can use your pesto on pasta, on bread, on baked potatoes, as a dip with vegetables, add it to other sauces, casserole, and stews, or any way you like.