Cavalier King Charles Spaniels are terrific little dogs - they are friendly, outgoing, and easy to train. However, if you're thinking of buying a Cavalier King Charles Spaniel, keep in mind that as a breed they have some significant health challenges.
Do your research.
You will be a better prepared buyer if you know what Cavalier King Charles Spaniel health issues are prevalent in the breed and what testing should be done to decrease the risk of your puppy growing up to exhibit those health problems.
The major health issues known at this time are mitral valve disease (heart), syringomyelia
(skull), patellar luxation (knees), and certain disorders of the eyes.
Find a breeder who tests for health.
There is one sure way to know whether you have found a caring and professional breeder in any breed of dog, and that is health testing.
If the breeder you are considering claims they don't test because "their line" doesn't have those problems, run away. No line of Cavalier King Charles Spaniels is free from the danger of these genetic problems. If the breeder doesn't even know what these disorders are, run away faster!
Both sire and dam should have been tested (and the breeder should be able to give you copies of the certifications) before they were bred. Ideally, the breeder will have several generations of certifications to offer you.3
Even with the best intentions and the most stringent of testing, something might go wrong. A good breeder will offer you a warranty (usually ranging from one to three years) against any significant genetic health problem your puppy might develop.
Make sure that the warranty does not require you to give your Cavalier King Charles Spaniel back to the breeder in order to claim your refund or replacement puppy - this tactic is both tacky and cruel. It is fine if the breeder requires your current dog to be spayed or neutered before offering a refund or replacement, however.