The function of reinsurance is to absorb the risks of the direct insurance industry. This has two main purposes: (i) reinsurance capital allows direct insurers to write more business, and (ii) it protects them against balance sheet fluctuations caused by large and unexpected losses. The reinsurance market is served by a relatively small group of some 200 professional reinsurers. However, throughout history a variety of alternative forms appeared that could be used to distribute risks beyond one insurer. Co-insurance, for example, was one of the main forms of secondary risk spread in the marine community for centuries. It dominated the London market and was, to a large degree, responsible for the late and restricted development of reinsurance companies in Anglo-Saxon markets. The emergence of ever-larger risks in the 20th century forced the industry to focus increasingly on dealing with large losses and capping the maximum exposures of insurers. This made the business more financial, a trend which received a new boost with the advent of insurance-linked securities (ILSs) in the 1990s. Since then, the market for alternative risk transfer (ART) has grown, not least with the advent of new investors such as different investment funds that provide alternative risk capital. However, towards the 2020s, professional reinsurers started gaining ground again after a series of large natural catastrophes and with the continuous rise of Asian economies. Since the 2010s, growth opportunities for reinsurance are sought mostly in emerging markets and by making more risks insurable. Emerging market growth, however, is challenging and the gap between insured and insurable economic losses is still widening. Since the turn of the millennium, the industry has invested in finding solutions to close this so-called protection gap. Professional reinsurers are also seeking to develop new markets by making emerging risks such as cybercrime insurable. Yet such dynamic risks are fundamentally different from older static risks. Solutions are sought in applying methods that already made natural catastrophes insurable, modelling techniques, and ART products.