Go to the Hill's website and look up the ingredients.
Hill's Pet Nutrition Products: Prescription Diet & Science Diet
For example, these are the ingredients in the Adult Small Bites:
Chicken, Ground Whole Grain Corn, Ground Whole Grain Sorghum, Ground Whole Grain Wheat, Chicken By-Product Meal, Soybean Meal, Corn Gluten Meal, Animal Fat (preserved with mixed tocopherols and citric acid), Brewers Rice, Chicken Liver Flavor, Soybean Oil, Flaxseed, Potassium Chloride, Iodized Salt, Calcium Carbonate, Choline Chloride, Vitamin E Supplement, vitamins (L-Ascorbyl-2-Polyphosphate (source of vitamin C), Vitamin E Supplement, Niacin, Thiamine Mononitrate, Vitamin A Supplement, Calcium Pantothenate, Biotin, Vitamin B12 Supplement, Pyridoxine Hydrochloride, Riboflavin, Folic Acid, Vitamin D3 Supplement), minerals (Ferrous Sulfate, Zinc Oxide, Copper Sulfate, Manganous Oxide, Calcium Iodate, Sodium Selenite), DL-Methionine, preserved with Mixed Tocopherols and Citric Acid, Beta-Carotene, Rosemary Extract.
Corn is a cheap filler and one of the top three allergy causing ingredients in dogs. It also contains wheat and soy, the other top two allergy causing ingredients.
This is the AAFCo definition for what is in chicken by-product-meal:
Chicken By-Product Meal - consists of the ground, rendered, clean parts of the carcass of slaughtered chicken, such as necks, feet, undeveloped eggs and intestines, exclusive of feathers, except in such amounts as might occur unavoidable in good processing practice.
AAFCO DEFINITIONS OF DOG FOOD INGREDIENTS
Pretty disgusting, huh?