There are two different categories of inappropriate urination, behavioral and physiologic. If they are standing, with tail lifted, peeing on walls or objects, then they are “spraying”, which is a form of territorial marking. This is a very difficult behavior to eliminate once it has become established, but there are options.
The second scenario is squatting and peeing, more like a normal “bathroom” posture. There are quite a few things that can be causing the problem. The first thing that you need to do is your cat in to the vet for a urinalysis. Cats can have urinary tract infections, just like human women. The condition is uncomfortable, making them feel as if they have to constantly go and it burns when they do. You may or may not see blood in the urine (hematuria).
The most common reason for inappropriate urination is dissatisfaction with the litter box situation. Most experts agree that you need to have one box per cat plus one. (I know, that’s a lot, and I have never done that, but hey, they’re the experts…). You need to make sure that the litter brand suits them, that the litter is cleaned regularly, and that the litter box is not in the vicinity of food and water bowls. The other thing that is vitally important is that you eliminate all traces of the urine that has been deposited – just a whiff stimulates cats to repeat the infraction. I would recommend one of the various enzymatic cleansers designed for pet odors. The enzymes actually digest the material.
If there have been changes in your household (i.e. travel), the cat may be expressing it’s stress and unhappiness. You will see disagreement among behaviorists as to whether cats attempt to “punish” owners, but there are plenty of people who strongly believe that an unhappy cat may urinate inappropriately to express dissatisfaction. Distraction, lack of access, and a thorough cleaning to remove scent “signals” will help.