Penetrant testing (PT) is one of surface methods of nondestructive testing, which is used to detect surface-breaking defects.

Penetrant testing requires the test surface to be properly prepared (any dirt, grease or paint needs to be removed, and defects need to be "opened") and special chemical products to be used:

• penetrant - a liquid with low viscosity and surface tension, which has the ability to penetrate even very narrow defects;
• developer - special product applied directly to the test surface; the developer draws penetrant from defects out onto the surface and provides better contrast, making it easy to see any flaw indications;
• cleaner/remover - effective pre-cleaning, post-cleaning and penetrant removal are crucial to the penetrant inspection process. 

The penetrant testing method is commonly used to inspect nonmagnetic materials, such as austenitic steels, magnesium, aluminum and nickel based alloys, titanium and some composites. There are two possible techniques for penetrant inspection: visible dye or fluorescent, in which the application of UV-A radiation is required.