For efficient testing of welded joints, it is indispensable to be acquainted with non-destructive testing methods, as well as to know as much as possible about the defects to be detected. In the welding literature there are comparatively few reports on lack of fusion defects, although the latter is of the most risky weld defects. For their detection, there are no efficient testing methods available. The present study focused on gathering as many as possible data on lack of fusion. It was found with which welding processes they were likely to occur, where they were located, and what their properties were. The indications of lack of fusion were studied using different non-destructive testing methods. Testing was carried out on butt-welded joints obtained in gas-shielded welding with a consumable electrode. In addition to non-destructive testing methods, metallographic and mechanical tests were used too. It was to be found why non-destructive testing methods were inefficient in lack of fusion detection.