Holiday testing is a non-destructive inspection method used to detect unacceptable discontinuities (voids, pinholes, fisheyes, cratering, nicks, and sometimes weaknesses) in a non-conductive coating film which are not easily visible.

Holiday testing is usually performed on tank interiors, chemical storage vessels and buried structures like pipe, because of the importance of maintaining adequate coating protection in aggressive service environments.

A holiday detector works by applying concepts of electrical conductivity. A non-conductive coating applied over a metallic substrate inhibits the flow of electricity between the detector probe and the base metal. Voids in the coating will allow exposed metal to complete an electrical circuit between the probe and ground, and results in a flow of electricity which is indicated by the detector. The choice of applicable holiday detector is influenced by coating type and thickness, substrate type, and work environment.