This paper treats a weld discontinuity called incomplete fusion in which fusion does occur between the weld metal and the fusion faces or adjoining weld beads. The paper states some reasons for the occurrence of the incomplete fusion, i.e. the arc-blow effect, an inappropriate gun position, an uncontrolled movement of the weld pool, a too-low energy input, an improper joint preparation. The defect is illustrated in some macrographs of the weld. At the end some practical conclusions are drawn.