For an insurance company, reinsurance is one of the tools used to mitigate the risks related to its activity. In reinsurance, the principle of pooling, which is intrinsic to the insurance business, takes on an additional dimension. Playing the role of insurer of companies, the reinsurer works in a global market and aims to help companies do their job better. Indeed, initially, insurance companies cede part of their risks in order to increase their underwriting capacity. Secondly, reinsurance contracts allow insurance companies to increase their solvency ratio without resorting to a call for capital. We will learn here the different technical aspects of reinsurance, the details of the treaties and their impacts on the balance sheet, the income statement and the solvency of the insurance company.