While you can use any type of lentils to make lentil soup, I prefer French green lentils, or lentilles du Puy. Prized by chefs for their earthy, peppery flavor and sturdy texture, these small lentils hold their shape when cooked. You can find them in many supermarkets—I usually get them at Whole Foods or Wegman's—or order them online. They work beautifully in soups as well as lentil salad. If you'd prefer to use common green or brown lentils, that's fine. Just watch the cooking time carefully as they can get mushy if overcooked.