Introduction - This article presents epidemiological data on the chikungunya outbreak in Mayotte and Réunion, along with considerations for assessing the impact of arboviral diseases. Methods - In Réunion, surveillance relied on active case-finding during periods of moderate incidence and on reports from sentinel physicians at the peak of the outbreak. Severe cases requiring hospitalization and deaths were subject to specific surveillance. In Mayotte, surveillance consisted of a passive reporting system based on physicians, supplemented by prospective case-finding of severe hospitalized cases. Additional investigations were conducted to verify the reliability of the results. Results - In Réunion, 266,000 cases were reported, along with nearly 250 severe cases, 44 cases of mother-to-newborn transmission, and more than 250 deaths. In Mayotte, the number of cases was revised to 60,000 based on additional investigations. Six severe cases and 9 cases of mother-to-newborn transmission were identified. Discussion - A review of entomological surveillance, international monitoring, modeling, evaluation of vector control, and communication appears necessary to better manage emerging arboviral diseases. (R.A.)