The International Association of Classification Societies (IACS) is an organization comprising of twelve marine classification societies with its headquarters in London.
Marine classification is a system for promoting the safety of life, property and the environment primarily through the establishment and verification of compliance with technical and engineering standards for the design, construction and life-cycle maintenance of ships, offshore units and other marine-related facilities. These standards are contained in rules established by each Society.
The members of IACS are:
- American Bureau of Shipping (ABS)
- Bureau Veritas (BV)
- China Classification Society (CCS)
- Croatian Register of Shipping (CRS) 
- Det Norske Veritas Germanischer Lloyd (DNV GL)
- Indian Register of Shipping (IRS)
- Korean Register of Shipping (KR)
- Lloyd’s Register (LR)
- Nippon Kaiji Kyokai (NK/ClassNK)
- Polish Register of Shipping (PRS) 
- Registro Italiano Navale (RINA)
- Russian Maritime Register of Shipping (RS)
In promoting maritime safety and clean seas, IACS and its individuals carry out research and development on marine-related topics, providing technical support and verifying compliance with published standards.
The classification process begins with the evaluation by the class society of a submitted design to determine its compliance with the rules. During construction, classification society surveyors attend the vessel to verify that it is built in conformance with the drawings and to the rules. On delivery, the vessel will receive periodic surveys by the society to verify that it is being maintained to the required standard. These surveys generally follow a five-year cycle of annual, intermediate and special surveys with the extent of the survey varying depending upon the age of the vessel or offshore unit and the type of survey being conducted.
For a vessel to remain ‘in class’ it must meet the class rule requirements at the completion of each survey. Should a vessel sustain in-service damage, the vessel’s owner advises the classification society of record so that a damage survey can be arranged. If the surveyor decides that the damaged vessel no longer meets the rules, the owner must carry out repairs to bring the vessel back into compliance if it is to remain in class.
That is the main reason why the class (or lack of a class) of a vessel is a crucial underwriting factor and why the Institute Classification Clause is found in most, if not all, marine cargo policies.