Although seminal research on counterproductive work behavior (CWB) included a seriousness dimension, there has been little investigation of the factors that determine seriousness perceptions and on the relevance of seriousness perceptions for responses to CWB (e.g., sanctions). Study 1 showed that organizationally-targeted CWBs were rated as more serious than interpersonally-targeted CWBs. Degree of harm and CWB motive (personal gain vs. prosocial) were not significantly related to seriousness ratings. In Study 2, minor CWBs resulted in less severe sanctions than more serious CWBs; this effect was greater for informal sanctions than for formal sanctions. Identifying the factors that determine perceptions of seriousness can develop our understanding of the nature of CWB, inform CWB prevention initiatives, and help prevent discriminatory application of sanctions.