There are a bewildering number of caging options, and as it is the single biggest financial investment you're likely to make in your new pets, it's an investment that should be considered carefully. 

Even though you should take your rats out of their cage for at least an hour a day for socialization and exercise, rats still require large cages. To get an estimate of the space needed, try a reliable cage size calculator. Keep in mind you'll need plenty of room for a litter box (if used) and other cage furniture, which we'll discuss in the next section.

Cages come in two basic types: aquariums with screen covers (usually the only viable choice for very young babies), and all-wire cages. Most people use the latter type for its superior ventilation and lighter weight. However, most wire cages sold as rat cages are frustrating for one reason or another, either because they're too small for the rats, or too hard for the owner to clean and access. Cages sold for ferrets and sometimes chinchillas are usually a much better choice. If available, select powder coating for the bars for its superior resistence to teeth and urine.

I personally use Midwest's Ferret Nation 142 and love them for their size, ease of cleaning, and terrific door design. However, there are many other suitable cages available.  Martin's makes a whole line of excellent cages geared toward larger cage pets. PetSmart's "All Living Things" brand offers a 30" long "Small Animal Cage" that's suitable for most young rats, and a 36" long "Penthouse" that works for rats of most ages. If you purchase a cage with an attached wheel, remove it, and replace it with a safe, suitable alternative (see the Exercise section).

One problem with most ferret and chinchilla cages is that the bars are spaced 1" apart, which allows young and some smaller adult female rats the opportunity to escape through the bars. If the rat can fit its head through the bars, it can fit its body too. If you're faced with owning young rats but wanting the large cage, try modifying the cage by adding 1/2" x 1/2" PVC coated hardware cloth attached by zip ties to the inside of the wires until the youngsters grow up. 

Rats are naturally curious, so place the cage in a convenient and well-trafficked area, unless your idea of a well-trafficked area involves hordes of screaming toddlers. Rats prefer to be kept at cool, but not cold, room temperatures, and should have excellent air circulation. I keep a small fan set on low or medium directed toward my cages. Soft, low volume music also helps entertain them. Be sure that the rats are in complete darkness at night. Constant exposure to light can stress them.