Depending on input material, process method, process parameters, and post-processing, the resulting defect state in as-built and finished additive manufactured (AM) parts can be highly variable and complex. To complicate matters further, the terminology used to describe specific defect types can be archaic or user specific and is in need of global harmonization. A common understanding of the root causes of defects and the effect of defects on relevant properties continues to evolve. In powder bed processing, for example, potential defects can be very small, down to the powder particle size. Defects also can occur because of single or multiple causes. Even when there are multiple causes, single defect types can be produced that fail by a single failure mode. Alternatively, a single defect type can have several different failure modes. The objective of this paper is to classify and identify types of technologically important defects that occur in AM parts produced by powder bed fusion (PBF) and directed energy deposition (DED). A breakdown of technologically important defects is presented in three sections: the cause, the defect, and detection by nondestructive evaluation (NDE). The effect-of-defect on relevant end-use properties is addressed whenever possible. For example, the effect of lack-of-fusion flaws on ultimate tensile properties and high cycle fatigue life is discussed, thus demonstrating the need to be able to detect such flaws. Thus, although the causes of the defects occurring in PBF and DED parts can be quite different, the actual defects can have some similarities. In general, reliable detection of defects by NDE does not depend on the process cause, but depends more on the size, geometry, and location (and, potentially, the morphology) of the defect as well as the complexity, density, and surface finish of the part.