Sets of quantitative and qualitative criteria that define “good” fusion and extrusion seams in HDPE geomembrane are presented as a common target for those developing seaming equipment and technology, and those developing nondestructive methods of seam evaluation. The criteria include seam peel and shear properties, impact resistance, and adjacent geomembrane stress rupture resistance. Qualitative criteria include microstructural observations of thin-slice plicrosections by transmitted light microscopy.