This Practice Note explains the purpose of aggregation clauses in insurance and reinsurance policies and discusses some of the relevant case law. It considers: what are aggregation clauses, why aggregate claims, 9/11 aggregation, aggregation arising from one event, aggregation arising from one originating cause, and aggregation resulting from a series of acts or omissions. This topic may be referred to as an introduction to aggregation of insurance claims and aggregation of reinsurance losses; defining or the definition of aggregation in insurance law; in the aggregate; and, aggregating language.