[Preface]. The 12 January 2010 earthquake was the latest and most devastating of many major sudden-impact natural disasters affecting Haiti in the last 10 years. It was also one of a series of sudden emergencies that mobilized the international community on a global scale. The response to Haiti, especially in the health sector, has been generous, even overwhelming. This internal and external response met considerable challenges and problems, some of its own making. As was the case in the response to the Indian Ocean tsunami and the Pakistan earthquake, not all those challenges were met effectively. The objective of this publication is to draw the lessons to be learned for improving the health response in future sudden-onset disasters. We know that massive earthquakes will occur again and some will devastate metropolitan areas or even the capital city, as was the case in Haiti. Haiti is the subject of this study, hopefully not the object, as Haiti has had her share of catastrophes. The scope of the book is limited to the health response, health being defined in its broad sense, not merely medical care or disease control. The review is confined to the immediate and early response in the first three months, the period during which most of the international assistance was mobilized and influences, for better or worse, rehabilitation and reconstruction. The publication focuses specially but not exclusively on those lessons that are of general interest, i.e., not specific to the special case of Haiti. The international community has much to learn from the response in Haiti where it has shown an ability to repeat its errors and shortcomings from past disasters. The methodology used for this study is common to most evaluations in-depth review of reports, evaluations, studies, and peer-reviewed scientific publications; over 150 interviews, half of them carried out exclusively for this study and others for similar evaluations carried out by one of the three authors; circulation of the draft to all interviewees for factual validation and comments on the authors’ interpretation of the findings; and, finally, discussion with a review board convened by PAHO/WHO.