Food bowls will need to be robust and heavy to prevent tipping.  Ideally an earthenware bowl should be used rather than plastic.

bowl.jpg

Rats are omnivores like mice, hamsters and gerbils.  In the wild they will usually eat vegetables and grains, but will also eat insects and meat.  Rats are fairly tolerant of changes in diets, but are prone to obesity if overfed.
Rats will eat to satisfy an energy requirement.  Commercial pet shop foods can vary tremendously in their quality.  Use a good rat food mix from pet stores, although in some cases a pre-prepared hamster mix will also give the same nutrients needed (not really recommended, though).  The diet can also be supplemented by a variety of treats, such as "Rodent Blocks" or "Lab Blocks".  Rats also appreciate their greens, although these should be introduced gradually and given in small quantities as they can cause diarrhoea.
Rats also like a bran or oat mash make up with milk as a supplement to their diet, especially good for pregnant and nursing mothers and young baby rats.
Protein supplementation is probably sensible for pregnancy or growth.  Female rats will gain 85g or 30% of their body weight during pregnancy and 60% of this weight gain is in the last week.
Adults will eat 15-25g of food per day.  Diets high in polyunsaturated fats can lead to Vitamin E deficiency with muscle weakness, eye discharges, irritability and death.  Because of this it is sensible to restrict the numbers of sunflower seeds and other grains that are fed.  Rats are especially keen on the flavours of cake and chocolate.  As very small treats, they can be used when training rats or socialising them with their owners.  Rats also seem to prefer black pepper, coffee and milk flavours to cheese.  Many vitamins, mineral and other micronutrient dietary deficiencies have been reported; sometimes these can take a couple of generations to become apparent.  See Nutrition for more information.  Some degree of coprophagia (or eating their own faeces) occurs naturally and is normal.

Infants

Baby rats, or kittens (some people even like to call them rittens!) need more calories, protein and fat than adults - just like a human baby.  They should also get more fresh foods, to help them grow up healthy.

Elderly/Sick Rats

Both sick rats and elderly rats will tend to lose weight, sick rats doing do quite rapidly.  In this case, feeding normal low fat foods won't give them the extras they need, so try giving them cat food, baby food, scrambled or boiled eggs, live full fat yogurt, bananas, avocados.  You can also try an energy booster, like Enervite, FerretVite or Nutrical, which give extra vitamins, minerals and calories.  One small pea sized amount a few times a weeks could help in keeping your rat that bit healthier.

Home Made Diets

Some people prefer to make their own home-made rat diets.  In fact, I have now started using one, and must admit that the rats really enjoy it, their coats are wonderfully soft and shiny, and it actually works out about as cheap as rat mix (barring the buying of the dog kibble/vitamins).
Here are a few links to some home-made rat diets:

Foods List

* Wash thoroughly first

Regular Foods

Apples* Corn Peaches*
Apricots* Egg yolk, cooked Peas
Blueberries Grapes (high in fat)* Peppers*
Bread Green beans * Porridge
Brussels sprouts Liver Raisins
Carrot Melon Squash
Cauliflower Mushrooms Tuna
Cherries* Oatmeal, cooked Yogurt (live culture)
Rat blocks Pasta  

Alternative Health Foods (Quite Regular Foods)

Aubergine - antibiotic properties Eggplant - antibiotic properties Parsley - helps prevent strokes
Bananas - antibiotic properties Garlic - antibiotic properties, helps fight arthritis Plums - antibiotic and anti-virus properties
Beans (cooked) (protects against tumours) Ginger - helps fight arthritis Prunes - antibiotic and anti-virus properties
Broccoli -protects against tumours Grapefruit seed - helps respiration, asthma Soybean protects against tumours
Clove (limited) - helps fight arthritis Mustard - antibiotic properties Strawberries* - anti-virus properties
Cranberries - anti-virus properties Olive leaf extract - helps respiration, asthma Tea  - antibiotic properties
Dates - helps fight arthritis Onion - antibiotic and anti-virus properties Tomatoes - protects against tumours

Limited

Alfalfa pellets Corn (dried) Spinach*
Avocado (high in fat) Cucumbers* Tofu, cooked
Beetroot Lettuce Turnip
Celery* Radish  

Foods To Give With Caution

Chocolate Coffee (known for causing cancerous tumours) Fizzy drinks

No Go Foods - Dangerous Foods

Mouldy/Off Cheese (mould is toxic to rats) Liquorice (suspicions of neurological poisoning) Raw Red Cabbage (contains anti-nutrients that destroys thiamin)
Green Potatoes (contains a toxin) Raw/Dry Beans (contain anti-nutrients that destroy Vit. A and digestion enzymes, causes red blood cell clumping) Raw/Dry Peanuts (contain anti-nutrients that destroy Vit. A and digestion enzymes, causes red blood cell clumping)
Green Bananas (inhibits digestion of starch) Raw artichokes (inhibits digestion of protein) Oranges/orange juice (can cause cancer in male rats)
  Raw Sweet Potato