As I mentioned in the Ingredient notes section above, I use brown lentils to make this lentil and potato soup. Brown lentils are the "typical" dried lentil you will find in most American grocery stores. Brown lentils work well in soups because they hold their shape somewhat (meaning, they don't totally break down into mush), yet still become soft and tender in the soup. If you like, you can also use green lentils; they have a very similar texture and cook time as brown lentils. For more info, check out this breakdown of the main categories of lentils and their ideal uses.