The International Society of Arboriculture (ISA) is an international non-profit organization headquartered in Champaign, Illinois USA. Its mission statement: "Through research, technology, and education promote the professional practice of arboriculture and foster a greater public awareness of the benefits of trees."
The ISA has over 20,000 members internationally, with international chapters and affiliates in Australia, New Zealand, UK, Canada, Asia, South America and Europe.
The ISA also runs the ISA Certified arborist program.
The ISA website offers a page where one can verify a Certified Arborist in a specific area.