Feline infectious peritonitis (FIP) is a dreaded and deadly viral illness of young cats. The disease is complex and still poorly understood which has made diagnostics and treatment a veterinarian’s nightmare since the discovery of the disease in the mid-1900s. Researchers are still actively searching for the exact mutation or mutations that allow the relatively benign feline enteric coronavirus (FECV) to become so virulent so quickly. Once the mutation is found, it should be much easier to test for the virulent strain and to develop an effective vaccine. Until then, veterinarians are left with several confusing diagnostic tests, none of which are totally accurate nor reliable. Currently, when a veterinarian can collect an effusion or lymph node aspirate, the fluorescent antibody test for FIP is one of the best, least invasive ways to obtain a convincing diagnosis. When a biopsy can be obtained or post-mortem examination performed, immunohistochemistry is considered to be the most definitive test. Having a firm diagnosis allows the veterinarian to feel more comfortable when giving owners the bad news about this difficult disease.