We need to talk to you about something serious. It appears, after much research and many conversations, that a lot of people have said the following in their lives, "You can either use Parmesan or Pecorino." Did your eyes bug out when you read that? Probably not. You've probably been desensitized by the myriad recipes, cooking shows, etc. that seem to think this makes sense. Let's talk about why it doesn't.
For starters, they are different cheeses -- not just sort of different, fundamentally different. They are both Italian, they are both usually found in an aged, hard, grate-able form (although there are also softer, fresher versions of Pecorino to be had). Those are about their only similarities. Parmesan is made from cow's milk, and Pecorino from ewe's milk (that's a lady sheep, guys). Parmesan is a little salty, fruity and floral. Pecorino is explosively salty, sharp and funky. Parmesan is perfect compliment to many other flavors. Pecorino IS THE FLAVOR, and will easily overpower anything you pair up with it. Pecorino is an amazing cheese, and we want to make sure you're using it in the best places possible, not just "if there's no Parmesan."
Pecorino Romano is the most commonly found version in the US (leading some people to just call this cheese "Romano," which really confuses everyone), but great Pecorinos also come from other parts of Italy. Seek out a Pecorino Sardo if you can find one, you will not be bummed out. If you've never used Pecorino on purpose, we want you to start with the first recipe below, Cacio e Pepe. This dish exemplifies Italian cuisine's incredible commitment to the simplicity of amazing ingredients. It's really pasta, cheese and pepper. If this dish doesn't kick off an obsession with Pecorino, let us know. We've got a few other tricks up our sleeves.