Non-destructive testing has no clearly defined boundaries. A simple technique such as visual inspection is a form of non- destructive testing, as also might be the measurement of an obscure physical property such as Barkhausen noise. It used to be considered that there were five major methods - radiographic, ultrasonic, magnetic, electrical and penetrant - but all these can be subdivided, and to them must be added a range of important new techniques such as acoustic emission methods, thermography and holography. Further, a large number of even more specialised methods have been investigated and found to have applications in particular limited fields; these are methods such as proton annihilation, neutron scattering, proton radiography, microwaves, and nuclear magnetic resonance.

In this paper, all the above stated methods are described in light of their advantages and disadvantages thus enabling the NDT professionals to judiciously choose a method for a particular analysis.