ISO 20121 (full name: ISO 20121:2012, "Event sustainability management systems –- Requirements with guidance for use") is a voluntary international standard for sustainable event management, created by the International Organization for Standardization. The standard aims to help organizations improve sustainability throughout the entire event management cycle. 

Every event – from a village barbecue to a major sporting event like the Olympics – will have economic, social and environmental impacts. Water and energy resources are put under pressure, significant amounts of waste and carbon emissions can be generated. Sometimes events can put a strain on local communities. By 2005, practitioners within the events industry were becoming aware of the need for more sustainable practices. 

Specifically, the Head of Sustainability at the London 2012 Olympic and Paralympic Games, David Stubbs, was looking for a way to make good on the sustainability promises made in the London Games bid. 

He raised the issue with the British Standards Institution (BSI) in the UK. This led to the creation of BS 8901:2007 "Specification for a sustainable event management system with guidance for use". After a period of review, the second version of BS 8901 was published in 2009. 

BS 8901 was received very positively by the international event industry, and was soon being widely used. For example, COP15, the United Nations Conference on Climate Change, was certified as compliant with BS 8901 in December 2009. The Microsoft Corporation achieved certification to BS 8901 at its Microsoft Convergence® 2009 event in New Orleans, Louisiana, in March 2009.
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Water and energy resources are put under pressure, significant amounts of waste and carbon emissions can be generated.