The most common are behavioral, urinary tract disease (infection, stones, kidney disease, neoplasia, etc.) or endocrine disease (diabetes, etc.). We always recommend consulting your veterinarian to identify what is going on. They will most likely suggest a visit which would include analyzing a urine sample, a thorough exam, and possibly blood work. Cats prefer a clean litter box so make sure to scoop it daily and completely change the litter weekly. There should be one extra litter box per number of cats. For example, if you have 2 cats, you should have 3 litter boxes. Some cats prefer different types of litter (clay vs. cedar, etc.), so that is also something to consider. Are there any changes in their home environment such as visitors, new baby, etc.? Again, there are many reasons a cat could be urinating outside the litter box, but a quick call to your veterinarian is the first step to finding out why.