The converging events that led to the devastation in Japan will raise a number of issues relating to the aggre­gation of losses under reinsurance contracts. The pre­cise wording of aggregation provisions in these contracts will be crucial in determining the scope of coverage. Unfortunately, much uncertainty remains as to the meaning of many aggregation provisions and ‘hours clauses’ in particular. This article highlights key points for reinsurers to consider in analyzing aggre­gation provisions in their contracts when faced with reinsured losses arising out of the Japan tragedy.