In this chapter we present an ontology design pattern to conceptualize complaints, an important concept still uncovered by ODPs. The proposed Complaint Ontology Pattern (COP) has been designed based on the analysis of free text complaints from available complaint datasets (banking, air transport, automobile), among other knowledge sources. We present a detailed use-case from consumer disputes. We evaluate the pattern by annotating the complaints from our use case and by discussing how COP aligns to existing ontologies. Knowledge engineers can further model complaints for specific domains and processes, satisfying different requirements via COP specializations.