The International Space Exploration Coordination Group (ISECG) was established in response to "The Global Exploration Strategy: The Framework for Coordination" developed by fourteen space agencies1 and released in May 2007. Several ISECG participating space agencies conducted a study of concepts for human exploration of the moon that allow individual and collective goals and objectives to be met. The 18-month study culminated with the development of the ISECG Reference Architecture for Human Lunar Exploration. The ISECG Reference Architecture for Human Lunar Exploration envisions how the space-faring nations of Earth can collaborate in exploring the Moon using the coordinated assets of many space agencies. It marks the first time that a group of space agencies has worked together to define a complex human exploration scenario. The reference architecture can be used to inform preparatory planning and decision-making within participating agencies. It represents a concrete step towards realizing the vision of the Global Exploration Strategy, which identified the Moon as one of the key destinations for future human space exploration. While pioneered for lunar exploration, this study can serve as a useful model for designing multilateral architectures to explore Mars and other destinations in the solar system. The ISECG Reference Architecture for Human Lunar Exploration involves a flexible, phased approach for lunar exploration that is designed to achieve significant exploration goals while recognizing global realities and challenges. It reflects the agency commitments to finding an effective balance between conducting important scientific investigations of and from the moon, as well as demonstrating and mastering the capabilities to send humans farther into the Solar System. In the reference architecture lunar exploration begins with a robust robotic precursor phase that demonstrates technologies and capabilities considered important for the success of the campaign. Robotic missions will inform the human missions and buy down risks. Human exploration starts with investigation of a polar region while demonstrating and validating the systems needed to take humans on more ambitious lunar exploration excursions. With confidence in the systems and capabilities, human and robotic exploration of the moon will proceed in phases providing the flexibility to adapt to discoveries or changing programmatic priorities.