1. A medical problem, possibly a urinary infection.
2. A bout of constipation, especially if the cat is older.
3. The box isn't cleaned often enough.
4. A change in litter brand, or box liners.
5. The box has been moved, and the cat doesn't like the new location.
6. Using litter box deodorizers that irritate a cat's senses.
7. It's a new box and it smells "funny."
8. The box was just cleaned with some harsh cleaners and the smell lingers.
9. Too much traffic. Most cats like a little privacy and quiet.
10. Too many cats using too few boxes. Rule of thumb: one box per cat.
11. Accidentally got locked out of the room.
12. A new cat won't let the resident cat(s) in, or vice versa.
13. The box is too far away. Rule of thumb: enough boxes on each floor to accommodate all cats.
14. A possible "turf issue" between two cats, and one won't share.
15. Strange cats are within "detecting distance"... that is, their scent is close by and the resident cat thinks the other cat is too near.
16. A male kitten has grown up enough to start spraying, so as he marks territory, resident cats may feel the need to "overmark." This is not a litter box issue. It's usually solved with a timely neuter... preferably before the behavior starts.
17. An unspayed female in the house is in heat and her odor makes males want to mark. Again, the solution is spay/neuter.
18. A cat just got declawed and the sand hurts his feet.
19. Suddenly the sand doesn't feel right for some reason (and only the cat knows what it is).
20. Something in the home has changed... new furniture? Rearranged? New baby? Someone left? Different work schedule? Loud arguments? Throwing things... etc.

Cats are very sensitive creatures and seem to depend on schedules and routines. They don't like things to change too much and may react by employing what we call "inappropriate elimination." The only trouble is, they don't know it's inappropriate.

They are simply responding to whatever is bothering them. But don't make the mistake of believing they are doing it on purpose, or as a revenge against you. Cats don't have such motives, and to think they do is simply our attempt at humanizing them in order to explain something WE don't understand.

That approach also leads to inappropriate punishment. Cats do NOT respond to punishment. It only creates a fear response, which could make them urinate out of the box even more.

They are like dogs in some ways... particularly in that they live in the moment. They may not be as forgiving as dogs, but only because they are smaller, as a rule, and more prone to hide for protection rather than to mollify an owner by licking and begging for affection.

If a cat is not using the litter box, go through the list and check off things that may or may not apply and try to address them accordingly.

Another point that could have been included is that another location may attract them away from the litter box. Perhaps another cat soiled some place, or perhaps the current cat soiled in that spot due to an accidental situation, and now it carries the smell of urine or feces. The same cat may use the litter box normally but ALSO soils in other locations.

The key here is to get rid of the odor that is flagging the spot... not an easy task. Cats have a fantastic sense of smell, so they can detect these things when we can't. They also produce some of the strongest smelling urine on the planet. This makes it extremely difficult to eliminate, especially if it has soaked into something that doesn't clean up easily. Often, diversionary tactics may become necessary, such as rearranging the furniture, or adding furniture, in order to cover the spot. Of course, you need to watch out for point number 20 and hope that isn't going to create new problems.

But the most frequent reason cats won't use the box is because of cleanliness. Some owners seem to think it's OK to scoop the box once a week, or even less. It's critical to keep all boxes cleaned at least once a day... preferably more often if there is more than one cat using a box. This alone will solve most litter box complaints.