Anyone can put a male and female rat together and get babies, but that is not what rat breeding is about
There is no shortage of rats on this planet, any rescuer will attest to that. Every day, hundreds of rats are euthanized because there are no homes available for them, so there is no reason to bring any more rats onto this earth unless we can be sure that by doing so, we're doing a service to the species.
This means breeding only the healthiest, long lived, even tempered rats.
Anyone breeding rats without the proper care and consideration is doing nothing more than simply multiplying. In fact, bad breeders are often referred to as 'multipliers' in rat communities!
But how do you know who is a reputable breeder and who is not?

As every body has their own morals, this page is merely a guide. I cannot make a decision for you on who you consider to be ethical; only you can decide that.
However, what I can do is tell you the most commonly agreed upon factors that make up a good, or conversely a bad, breeder.
These are the things I personally would consider when checking out a breeder.

A few things to note beforehand: The NFRS is the rat equivalent of the kennel club here in the UK. They are the people who set down the guidelines on how a show rat of any variety should look, and they also keep a list of registered breeders which you can get by emailing breeders@nfrs.org
However, do not be put off if the breeder you're interested in is not registered with the NFRS, or another rat club. This does not necessarily mean they are not doing things properly, and similarly, just because someone is registered, it doesn't necessarily mean they'll be doing things right. This is why it is so important to thoroughly vet a breeder before doing business with them.

Please note - The rat community in the UK is quite small, and as such, most people in the rat fancy know of each other. Like with a lot of animal communities, there can be drama, rumour-spreading, lying and nastiness floating about.
I have known breeders have out and out lies spread about them by someone they upset over something not even related to rats, and even have threats made against them by other rat people.
I've seen breeders I know first hand to be exceptional, publically slandered on rat forums.
While I’m not trying to give the impression it is not a nice community most of the time, you do need to be aware and accepting of the fact that not everything you hear about a certain person or breeder will be true.
Unsubstantiated rumours run rife in the rat fancy, sadly. This is why it is even more important to hear and see things for yourself, and vet breeders personally, rather than relying on hearsay and rumours.

Also, do not be put off breeders by talk of showing. You do not need to be interested in showing to go to a breeder for your rats.

So, a few things to think about now you've located a breeder you're interested in, and spoken to them:

Were they happy to discuss their breeding practises with you?
A good breeder will be happy to tell you all about their breeding practises and their husbandry methods. Good breeders are only too happy to be asked lots of questions, as it shows them that the person enquiring is serious about getting some rats and doing things properly. Avoid breeders who are reluctant to answer questions, or skirt around the issue of husbandry, or are defensive when you ask questions about their rat care. If they are happy in their mind that they're doing everything right, they should have no reason to be defensive when questioned.

Why are they breeding?
Perhaps the most important question.
I would even go as far as to say that if the answer is anything other than 'to improve the health, and thusly welfare, of the rat as a species', be very wary.
Anyone who breeds to make money, or because they like the experience of watching baby animals born, or to 'educate the kids' is likely not holding rat welfare as their main priority. A good breeder should have a passion for rats and rat welfare, and a desire to see the species live longer, healthier lives.
A good breeder almost always loses money by breeding, as the cost of raising litters outweighs whatever money you will make back when selling the babies. It should be something that is done selflessly to aid the rat population, never something done purely for the owner's benefit.

How many varieties do they breed and how many litters do they produce each year?
Good breeders tend to focus on only two or three varieties of rat, just like good dog breeders tend to only stick to one or two breeds.
This is because working with too many varieties creates a 'jack of all trades, master of none' situation. Good breeders always focus on quality over quantity. Be extremely cautious of any breeder who says anything like 'I breed blues, roans, dumbos, rexes, hairless, silvers, burmese, all kinds of rat!' as almost always, these are not ethical breeders but more like rodent mills who churn out as many rats as possible in as many fancy varieties as possible, without ever giving a thought for health or temperament or longevity.
You would be very suspicious if a dog breeder told you they bred poodles, dobermans, yorkies, staffies, labradors and collies all at the same time, and the same caution should be used when vetting a rat breeder.

Similarly, be wary of anyone who breeds more than 2 or 3 litters a year, especially from the same doe! Good breeders will only have a few litters a year, and will not breed from a single female more than twice. And even then, she will only be bred from twice if she is truly exceptional!

How much do they charge for their rats?
Be wary of anyone who charges excessive amounts of money for rats, particularly if they do so while telling you the rat is a 'rare' or 'unusual' variety.
Without exception, these will be people out to make a fast buck from someone's ignorance or naivety.
The standard price for a rat from a breeder is around £10. Anything much more than that should ring alarm bells.
Hairless rats are a good example of a variety that is often sold for up to twice that amount by people who are trying to cash in on a novelty.
There is absolutely no logical reason why a topaz rex dumbo should sell for more than a top eared black hoody; both cost as much to breed and maintain, and anyone who would try to charge more for a rat that is a 'rare' variety is only concerned with their pocket and how much cash they can suck out of gullible people, not the rat's welfare.
It’s worth bearing in mind that there isn't much that is 'rare' in the world of rats. I've owned a whole rainbow of varieties, ear sets, coat types, and even a handful of hairless, all of whom have come to me for free via rescue.
Some breeders will even give babies away for free, particularly to returning buyers or people that they know, because their priority is getting their rats a good home, not making money.

Choosing a good breeder is all about common sense, really.
Use the same logic you would hopefully use if buying a puppy: choose someone knowledgeable, some dedicated to the animal and not money, someone who keeps records of their breeding stock's family history, someone who is warm and loving toward the animals rather than cold and robotic, someone who has a history of breeding good, healthy, well tempered animals.
If you do not vet a breeder well enough, buying animals off them is as bad, or worse, than going to a pet shop. There is a huge problem with 'back yard breeders' putting two rats together and selling them in the local paper for a quick buck, and a lot of people don't realise the harm these kinds of people do to the rat population.
When it is animal welfare that’s the issue, there are no standards too high.