Introduction The Chikungunya virus is an alphavirus of the Togaviridae family transmitted by mosquitoes of the genus Aedes (Ae. aegypti, Ae. albopictus, Ae. polynesiensis). This virus is primarily found in Africa, Southeast Asia, and the Indian subcontinent. Between early January and mid-May 2005, a Chikungunya epidemic raged for 19 weeks in the Comoros, marking the virus’s first emergence in the southwestern Indian Ocean. Starting in mid-April, suspected cases imported from Grande Comore were reported in Mayotte, and the outbreak reached Mauritius by the end of April. The first cases were reported in Réunion by the end of April. Monitoring this outbreak in Réunion required the implementation of a scalable surveillance system. The objectives of this surveillance were to describe the outbreak in terms of time, location, and individual characteristics; to guide prevention and intervention measures; and finally, to identify avenues for research. Methods Surveillance in the general population was based on active case finding during periods of low incidence. During the hyper-epidemic phase, the number of new symptomatic cases was estimated by extrapolating data from a network of sentinel physicians. These data were also cross-checked against a range of other indicators to ensure consistency (reports from physicians outside the network, emergency department visits, hospitalizations, sick leave, and self-reported cases). Emerging hospital cases were investigated. Death certificates and overall mortality were monitored. Results After a weekly peak of 450 cases in May 2005, the incidence stabilized at around 100 cases per week during the southern hemisphere winter before rising sharply at the end of December to reach an estimated peak of over 47,000 cases during the 5th week of 2006.As of May 14, 2006, nearly 34% of Réunion’s population had been affected by Chikungunya. Two hundred sixty-two1 severe cases were recorded. Forty-five mother-to-child transmissions were reported,1 including 10 cases of meningoencephalitis in newborns and 1 death. Two hundred nineteen death certificates mentioned Chikungunya, and a significant excess mortality coinciding with the 2006 epidemic peak¹ was identified. Discussion The emergence of Chikungunya in Réunion triggered a prolonged epidemic of exceptional magnitude, which underwent several phases of varying incidence levels, requiring adjustments to the surveillance system. Severe forms of the disease, cases of mother-to-newborn transmission, and deaths associated with Chikungunya have been described. This Chikungunya epidemic once again highlights the need to promote a regional strategy for mutual information sharing, prevention, and control of arboviruses in particular.