Reinsurance is insurance that an insurance company purchases from another insurance company to insulate itself (at least in part) from the risk of a major claims event. With reinsurance, the company passes on ("cedes") some part of its own insurance liabilities to the other insurance company. The company that purchases the reinsurance policy is called a "ceding company" or "cedent" or "cedant" under most arrangements. The company issuing the reinsurance policy is referred to as the "reinsurer". In the classic case, reinsurance allows insurance companies to remain solvent after major claims events, such as major disasters like hurricanes and wildfires. In addition to its basic role in risk management, reinsurance is sometimes used to reduce the ceding company's capital requirements, or for tax mitigation or other purposes.

The reinsurer may be either a specialist reinsurance company, which only undertakes reinsurance business, or another insurance company. Insurance companies that accept reinsurance refer to the business as "assumed reinsurance".

There are two basic methods of reinsurance:

1. Facultative Reinsurance, which is negotiated separately for each insurance policy that is reinsured. Facultative reinsurance is normally purchased by ceding companies for individual risks not covered, or insufficiently covered, by their reinsurance treaties, for amounts in excess of the monetary limits of their reinsurance treaties and for unusual risks. Underwriting expenses, and in particular personnel costs, are higher for such business because each risk is individually underwritten and administered. However, as they can separately evaluate each risk reinsured, the reinsurer's underwriter can price the contract more accurately to reflect the risks involved. Ultimately, a facultative certificate is issued by the reinsurance company to the ceding company reinsuring that one policy, and is used for high-value or hazardous risks.
2. Treaty Reinsurance means that the ceding company and the reinsurer negotiate and execute a reinsurance contract under which the reinsurer covers the specified share of all the insurance policies issued by the ceding company which come within the scope of that contract. The reinsurance contract may obligate the reinsurer to accept reinsurance of all contracts within the scope (known as "obligatory" reinsurance), or it may allow the insurer to choose which risks it wants to cede, with the reinsurer obligated to accept such risks (known as "facultative-obligatory" or "fac oblig" reinsurance). These types of contracts are typically long-term.

There are two main types of treaty reinsurance, proportional and non-proportional, which are detailed below. Under proportional reinsurance, the reinsurer's share of the risk is defined for each separate policy, while under non-proportional reinsurance the reinsurer's liability is based on the aggregate claims incurred by the ceding office. In the past 30 years there has been a major shift from proportional to non-proportional reinsurance in the property and casualty fields.