If your cat won't use a litter tray, there is likely to be an obvious reason. 

Here are the most common causes and some solutions to try:

Dirty litter trays: Cats don't like using a tray if it is heavily soiled.
Solution - Litter trays should be cleaned out completely every couple of days and topped up with fresh litter daily once the solids and clumps are removed.

Put off by litter:
Solution - Using scented litter, deodorants or disinfectants with strong smells may put the scent-sensitive cat off using the tray. Use a mild detergent and hot water or disinfectant specifically recommended for tray cleaning. Cats learning to use the tray need to establish it as the appropriate toilet site and too frequent cleaning may weaken the association.

Wrong type of litter:
Solution - Changing the consistency or type of litter may put the cat off using it. Hard wood-based pellets may have been acceptable as a kitten but as cats get heavier there are some that object to walking on the uncomfortable surface. Cats prefer fine grain litter with the consistency of sand with no scented deodoriser. If you want to change the type you use, mix the new one in gradually over a week to gauge the cat's reaction.

Position of the litter tray:
Solution - If the tray is positioned in the open where a dog, children or other cats disturb it, the cat may feel too vulnerable to use it. Instead they may seek a more secure spot behind the television. Cats may not like to use a tray if it is next to a noisy washing machine or tumble dryer. Place the tray in a quiet corner where the cat only has to watch in one or two directions at once rather than in the open or in a thoroughfare. Placing food near the tray will put cats off using it so place feeding bowls elsewhere.