Roughly chop arugula, and place in food processor with garlic and pine nuts. Process a few seconds, then scrape the top. Process again, while streaming in olive oil.
Remove to a bowl and stir in Parmesan cheese. Use right away, or keep in the refrigerator for 2 to 3 days.
Note: If using on pasta, put pesto in a large bowl first before adding hot cooked pasta to the bowl. Toss to combine, and add more Parmesan, to taste.