It is likely that at some point, you will need to introduce new rats to your existing colony. How difficult this is depends hugely on whom is being introduced to whom. Introducing baby rats to adults is fairly easy. Introducing two adult bucks to one another can be quite hard, and on very rare occasions it can be impossible. The important thing to remember is to never just dump two new rats in with your existing rats from the offset, because your resident rats will always defend their territory. This can be at best traumatic for the new comers, at worst, lethal. Introducing new rats can take as little as a day, or as long as several weeks.

Introducing rats
Introducing two baby rats to one another is child's play.
Rat kittens will readily accept other rat kittens from different litters with no problems. Introducing baby rats to adult rats can sometimes be a little more involved, but its certainly possible, and often just as easy as introducing two youngsters to one another.
When you consider getting a new rat kitten to introduce to your existing rats, it's always better to get two babies. This is because they will make company for one another and the entire experience will be less nerve-wracking for them if they have a friend for moral support. Also, in the rare event that the new rats are never accepted by the existing colony, they will need to live in a seperate cage and obviously it's better that theres two of them than having one rat live alone. It's also important to ensure the baby rats are at least 6 weeks old before you try and introduce them as adult males will sometimes kill strange baby rats younger than this.

The first thing to do when you bring your new babies home is think about quarantine. See this page for more info on this.

Some adult rats will accept a new baby from the offset.
I have done several intros where baby rats were accepted into the group within the hour. It depends very much on the personality of the rats involved. Some will simply accept the newbie quite happily and curl up to sleep with them, while others will chase them around and harrass them. You will know your own rats best and how you think they will react. I will often hold the new babies up to the bars and let my resident rats sniff at them. You can usually tell from their reactions how they feel about the newcomers. If they are busily sniffing and seem interested, this is a good sign. If they puff their fur, hiss or try to lunge at the new baby, this is an aggressive action and might suggest you will need to take things slowly
If this is the case, there is a process to introduce new rats which is commonly used and has a good success rate.

Put your new rats into a seperate cage which is close enough to the adults cage that they can smell and hear the new comers, but cannot reach through the bars and grab them! It's probably best to leave them in this cage for the first day or so so they can settle in and the adults will have plenty of time to get used to their smell.
The next step is to do a cage swap. Put the new rats in the adults cage, and put the adults in the new rats cage. Leave them in there untill they no longer seem interested in exploring. This lets each group of rats get used to being on the other's territory. Some people advise you spend several days just switching cages like this but it really depends on your rats reactions. If your existing rats don't seem overly bothered by the smell of the newcomers, thats a good sign. If any of your rats puff their fur up or show agressive signs just at the smell of a new comer, then you'll want to take it slower.

When you feel happy with the cage swaps, then you can move on to the first actual meeting.
Cover all your rats, the resident ones and the new ones, with vanilla essence. This makes sure they all smell the same so the resident rats are less likely to view the new ones as intruders.
Put all the rats somewhere neutral. This should be somewhere that neither group of rats view as their territory. A popular choice is the bath tub, or the sofa. This means that neither rat feels he has a territory to defend, and also that all the rats will be more preoccupied with exploring the new turf than attacking one another.

It's common for the rats to completely ignore each other at this point and just wander around seemingly oblivious to one another. But the rats should be left together until they begin to notice one another. In my experience, this takes around 10 minutes.
It's likely that the first sign you will see of the adult rats noticing the new ones is some power grooming. Power grooming is when one rats grooms another, but hard. It's a rat's way of saying 'well, I accept you so Im going to groom you, but I want you to know who's boss, so Im grooming you hard'. It's likely that the subordinate rat will squeak during this process, because it's a little nerve wracking and uncomfortable for any new rat, but rest assured that it's perfectly normal and you shouldn't intervene.
The general rule of intros is 'no blood, no foul'. As long as the rats are not agressively attacking each other to the point where blood is shed, they are doing well and should be left to sort it out. It's always tempting to rush in and split up the rats, especially if one is a youngster and the other is a big adult, but by interferring too much, you can actually pro-long the process. Rats need to sort out who is who in order for their group to run smoothly. If you don't allow them to do this, it can make things worse.

If you do need to split the rats up, such as if there is some serious agression and bloodshed, it's important to remember not to just stick your hand in between the two. This could easily result in you being bitten or scratched, and take it from me, rats can do a lot of damage to human hands in this situation. A much better idea is to have a thick towel on standby. If things get hairy, wrap your hands in the towel before plunging in there. Another good idea is to have a squirt bottle filled with water on hand and to give a quick squirt when things get nasty. This will usually make the rats stop what they're doing immediately, and sometimes make them pause to wash.

If things go well, you will need to repeat this step a couple of times or untill you feel confident with how the rats are getting on. Play this by ear and take it as it comes. There are no concrete rules. It's all down to how you think things are going. If this step doesn't go well, then take a step back. Return to the cage swaps and keep doing this for a little longer. Once you've passed the bath tub stage successfully and everyone seems to be getting along, it's time to put the newcomers into the resident rats cage. Firstly, clean out the cage thouroughly, including all toys and hammocks. It might be useful to dab a little vanilla essence around too.
Place all the rats in the cage.
It is likely that at this point, no matter how well previous introductions have gone, there will be some squabbling. This is perfectly normal and, as with the bath-tub stage, should be allowed to run it's course as long as there is no blood. I usually remove igloos and any boxes so that the resident rats can't box the newbies into a corner. Leave the rats untill they are no longer bothering with one another.

Even if this stage goes well, it's not advisable to leave the newcomers in the cage unsupervised at this point. Whenever you are not in the room, or not actively watching them, put the babies back in their own cage. Within a few days or regular meetings in the main cage, you should be able to leave the babies with the other rats quite happily. Remember that if a rat really doesn't like another rat, he'll take action as soon as possible. If your rats were going to take exception to one another, they would do so within the first few minutes of meeting. If you've gotten through all these stages relatively calmly, you can be fairly sure that everything will go well from now on. Be aware that some dominance scuffles and power grooming will happen for a while, just to remind the babies where in the pecking order they are, and to bring them into line if they play up, but this is perfectly normal and is just like us telling a toddler off for being cheeky or disobediant! As soon as you see all the rats curled up asleep in a big pile together, you'll know everything is fine.