You may be wondering which major environmental and conservation organizations are the most powerful and prominent at the international level and have an international presence in many countries around the world with actual offices, chapters, affiliates and/or partner organizations in those countries.  These organizations are the following (not necessarily in order of size or importance):

The WWF www.panda.org headquartered in Gland, Switzerland

Friends of the Earth International www.foei.org headquartered in Amsterdam, The Netherlands

IUCN - The World Conservation Union www.iucn.org headquartered in Gland, Switzerland

Greenpeace International www.greenpeace.org/international headquartered in Amsterdam, The Netherlands

Conservation International http://conservation.org headquartered in Arlilngton, Virginia

The Nature Conservancy http://nature.org headquartered in Arlington, Virginia