Dogs that have been trained to go to the potty, particularly to urinate, outside may suddenly start peeing indoors for any one of many reasons.  The dog is probably suffering some manifestation of a medical, psychological or behavioral problem that you need to be aware of, therefore you should regard this new development compassionately and do all you can to get to the root cause of the new and unwanted behavior.  Many veterinarians and professional dog trainers assert that pet owners should have their pets professionally evaluated before assuming they can eliminate the problem by using discipline. Once any medical conditions have been ruled out, a pet behaviorist can be consulted to help find out what could be going on in your dog’s mind and to correct the behavior.-Canine diseases such as diabetes can, just as it does in humans, cause a dog to increase its water intake. This in turn leads to more potty breaks, and sometimes the dog’s bladder fills up quickly, and the dog cannot wait for the next walk. Sometimes, you will notice that a sudden onset of incontinence takes place shortly after the addition of a new medication. Calling the prescribing veterinarian may help you to feel better about this in cases where temporary incontinence is a side effect of the medication.- Sometimes, dogs who are in a state of panic or who are facing severe separation anxiety, which is common for pets when they are first left alone, can bring about more frequent and inappropriate urination indoors. Separation anxiety usually exhibits alongside other behavioral clues like chewing or clawing at doors and windows or other openings to the home or places where the animal has seen their owner come and go from. Your dog may also develop anxiety due to something that may have happened in your absence, such as a loud sound from a thunderstorm, or a brief power outage.  Any number of things could be the cause.- In addition to these other potential reasons, male dogs tend to  urinate to mark their territory. They will mark on walls, chairs and table legs or other vertical surfaces both indoors and around the outdoor perimeter of the house and yard. Any time a male dog is introduced to a house, his instinct is to mark the territory.  This behavior can be gradually tapered and discontinued with the right training, so always consult a professional before trying methods on your own that may do more harm than good.