If your problem in any way involves AGGRESSION, it is imperative that you consult a well qualified behaviorist or veterinary behaviorist who has a lot of experience dealing with aggressive dogs, some sources for which are given on my Farewell page.
Even if you have never yet felt dissatisfied with any dog's behavior, it is well to be acquainted with problem prevention and problem solving protocols.
Please also look at the Canine Nature sub-topic of the Training topics (which I will try to keep duplicated here), as many objectionable behaviors are actually perfectly normal canine behaviors that in Nature would promote the dog's survival, but that in our human Society need to be re-directed into acceptable outlets. Also please look at the Puppy Rearing topics, because many problems can be prevented by well thought out puppy rearing proceedures.
This section deals only with behavior issues. For health issues, see the Health Issues sub-topic of the DOG CARE section. (Thanks to one of my site users for pointing out that some people would look for health issues under Problems.)