Dogs are taken to the veterinarian for skin problems more than any other reason. Most often they are itchy and that itchiness is typically due to allergies. The text of canine and feline dermatological diseases is large because there are hundreds of skin diseases, however, it can help to start with some generalities when determining the cause.
Again, there are many causes of itchiness, but allergies are one of the most prevalent causes. The most common allergies are flea allergy, food allergy and atopy, which is an allergy to environmental allergens, especially pollen, molds, and house dust mites (which are in all houses). The location of the lesions and itchiness can give clues as to the type of allergy.
Flea allergies are very common. With this allergy, pets are most itchy in the back half of the dog. Commonly there is hair loss, redness, and possibly scabs and infections over the dog's hips and tail base. You may not even see fleas! If a pet is allergic to fleas, all it takes is one flea to make the dog incredibly itchy. When the pet is licking and chewing at itself, he or she often eats the flea which then destroys the evidence. The flea bite has still set off an inflammatory response so the pet is still itchy. Veterinarians seeing dogs with lesions in these areas will suspect a flea allergy even if they don’t see any fleas.
A food allergy is also a very common allergy. Dogs show food allergy symptoms in their ears, face, feet, or rear. Pets don't become allergic to a brand, they are allergic to one or more ingredients. Thus just changing brands of food probably won't help the itchiness. To make matters more difficult, research shows that that in the manufacturing process of pet foods, there is a lot of cross contamination of ingredients. This means there are ingredients in the food that are not on the label. There is no valid blood or skin test to determine if your pet has a food allergy, but your veterinarian can help you find a food to use as a test (commonly this is rabbit and potato by Royal Canin as this is manufactured for food trials since there are no other ingredients.)
Atopy, the allergy to such environmental things as pollen, house dust mites, and molds, frequently causes the dog to be itchy in the face or feet. Unfortunately this type of allergy is harder to control because it is impossible to totally remove these items from the pets life. Luckily many dogs can be helped with hyposensitization by a veterinary dermatologist or by the drug Atopica.
Skin lesions from allergies are frequent as well. These generalities are not hard-and-fast rules, but can used as a starting point. Your veterinarian will also check for any secondary skin infections as these will make the dog itchy even if you get rid of the allergen from the dog's environment. Your veterinarian can help your itchy dog, since this the most common complaint they see, they have lots of experience!