Background: Mass gatherings present enormous challenges for emergency preparedness. Planners must anticipate and prepare for communicable and non-communicable disease outbreaks, illnesses, and injuries to participants, crowd control, and disaster responses to unforeseen natural or man-made threats. The Hajj, the largest annually recurring mass gathering event on earth. It attracts about 3 million pilgrims from over 180 countries who assemble in Mecca over a 1-week period. Methods: A literature review was conducted using Medline and OVID, while searching for published data concerning human stampedes and crowd control measures implemented to prevent human stampedes. The review was further extended to include media reports and published numbers and reports about Hajj from the Saudi Arabian government, in both the English and Arabic languages. Results: Because millions of pilgrims undertake their religious ritual within strict constraints in term of space and time; this rigour and strictness have led to a series of large crowd disasters over several years, thus putting pressure on the authorities. In the past few years, the government of Saudi Arabia have put an enormous effort to solve this difficulty using state of the art innovative scientific means. The use of crowd simulation models, assessment of the best ways of grouping and scheduling pilgrims, crowd management and control engineering technologies, luggage management, video monitoring, and changes in the construction of the transport system for the event. Conclusions: A large gathering such as the Hajj still holds an increasing risk for future disasters. International collaboration and continued vigilance in planning efforts remains an integral part of these annual preparations. The development of educational campaigns for pilgrims regarding the possible dangers is also crucial. Lessons gleaned from experiences at the Hajj may influence planning for mass gatherings of any kind, worldwide. (C) 2016 Elsevier Ltd. All rights reserved.