Determine if the problem is inappropriate urination or spraying.

Urinating on carpet, clothing, etc. usually indicates a behavioral or medical problem.
Spraying or urine marking is usually done by males and indicates territorial marking. Neutering decreases this tendency.

In Multi-cat homes, make sure you know which cat is causing the problem.

Medical vs Behavioral

Inappropriate urination can signal a health problem. Symptoms include:

Blood present in the urine
Urinating or defecating more or less than usual
Straining when in the litter box
Drinking more or less than usual
If your cat exhibits any of these signs, a Urinalysis/Fecal and/or Physical exam are necessary

If your cat is spraying possible reasons could include:

Too many cats in the household
Your cat needs to be neutered (if already neuterd, call the vet)
Changes in your cat's environment (a new baby, new animals-inside and outside of the house, moving, house guests, etc.)

Inappropriate urination/defecation on horizontal surfaces outside the litter box usually indicates litter box aversion. Reasons could include:

Urinating on specific areas can also be anxiety based:

If this is the case, expect to other behavioral changes (hiding, avoidance, aggression)

Treatment

Treatment of House Soiling involves:

Recommended action for behavioral soiling:

Be sure to have at least one more litterbox than number of cats in the household
If you have a multiple-level house, be sure that at least one box is located on each level of the house.
Use open litterboxes-no hooded ones.
use litterboxes that are at least 1.5times the length of yor largest cat.
Use a maximum of 2-3 inches of unscented, clumping litter.
Scoop soils from each box twice daily. Completely empty and rinse each litter box once per week. When rinsing, use water and a mild, unscented detergent.
if these modifications do no produe the desired response, consider trying Feliway Plug-ins.