The article concerns the time of flight diffraction testing technique (TOFD), which is, next to the simultaneous TOFD + Phased Array testing, one of the most effective methods of volumetric non-destructive tests. The article discusses the advantages of the TOFD technique as well as the basis of diffraction phenomenon and the formation of imaging signals. In addition, the article presents a TOFD image of a welded joint and describes its characteristic elements. Also, the article discusses the TOFD-related testing standards and analyses their requirements related to welded joints and their acceptance criterion, i.e. the quality level according to PN-EN ISO 5817. The target readers of the article include NDT personnel, inspectors, welding engineers and welding equipment manufacturers wishing to implement an effective tool enabling the detection of welding imperfections.