Problem/Condition: In 2012, the number of international tourist arrivals worldwide was projected to reach a new high of 1 billion arrivals, a 48% increase from 674 million arrivals in 2000. International travel also is increasing among U.S. residents. In 2009, U.S. residents made approximately 61 million trips outside the country, a 5% increase from 1999. Travel-related morbidity can occur during or after travel. Worldwide, 8% of travelers from industrialized to developing countries report becoming ill enough to seek health care during or after travel. Travelers have contributed to the global spread of infectious diseases, including novel and emerging pathogens. Therefore, surveillance of travel-related morbidity is an essential component of global public health surveillance and will be of greater importance as international travel increases worldwide. Reporting Period: September 1997-December 2011. Description of System: GeoSentinel is a clinic-based global surveillance system that tracks infectious diseases and other adverse health outcomes in returned travelers, foreign visitors, and immigrants. GeoSentinel comprises 54 travel/tropical medicine clinics worldwide that electronically submit demographic, travel, and clinical diagnosis data for all patients evaluated for an illness or other health condition that is presumed to be related to international travel. Clinical information is collected by physicians with expertise or experience in travel/tropical medicine. Data collected at all sites are entered electronically into a database, which is housed at and maintained by CDC. The GeoSentinel network membership program comprises 235 additional clinics in 40 countries on six continents. Although these network members do not report surveillance data systematically, they can report unusual or concerning diagnoses in travelers and might be asked to perform enhanced surveillance in response to specific health events or concerns. Results: During September 1997-December 2011, data were collected on 141,789 patients with confirmed or probable travel-related diagnoses. Of these, 23,006 (16%) patients were evaluated in the United States, 10,032 (44%) of whom were evaluated after returning from travel outside of the United States (i.e., after-travel patients). Of the 10,032 after-travel patients, 4,977 (50%) were female, 4,856 (48%) were male, and 199 (2%) did not report sex; the median age was 34 years. Most were evaluated in outpatient settings (84%), were born in the United States (76%), and reported current U.S. residence (99%). The most common reasons for travel were tourism (38%), missionary/volunteer/research/aid work (24%), visiting friends and relatives (17%), and business (15%). The most common regions of exposure were Sub-Saharan Africa (23%), Central America (15%), and South America (12%). Fewer than half (44%) reported having had a pretravel visit with a health-care provider. Of the 13,059 diagnoses among the 10,032 after-travel patients, the most common diagnoses were acute unspecified diarrhea (8%), acute bacterial diarrhea (5%), postinfectious irritable bowel syndrome (5%), giardiasis (3%), and chronic unknown diarrhea (3%). The most common diagnostic groupings were acute diarrhea (22%), nondiarrheal gastrointestinal (15%), febrile/systemic illness (14%), and dermatologic (12%). Among 1,802 patients with febrile/systemic illness diagnoses, the most common diagnosis was Plasmodium falciparum malaria (19%). The rapid communication component of the GeoSentinel network has allowed prompt responses to important health events affecting travelers; during 2010 and 2011, the notification capability of the GeoSentinel network was used in the identification and public health response to East African trypanosomiasis in Eastern Zambia and North Central Zimbabwe, P. vivax malaria in Greece, and muscular sarcocystosis on Tioman Island, Malaysia. Interpretation: The GeoSentinel Global Surveillance System is the largest repository of provider-based data on travel-related illness. Among ill travelers evaluated in U.S. GeoSentinel sites after returning from international travel, gastrointestinal diagnoses were most frequent, suggesting that U.S. travelers might be exposed to unsafe food and water while traveling internationally. The most common febrile/systemic diagnosis was P. falciparum malaria, suggesting that some U.S. travelers to malarial areas are not receiving or using proper malaria chemoprophylaxis or mosquito-bite avoidance measures. The finding that fewer than half of all patients reported having made a pretravel visit with a health-care provider indicates that a substantial portion of U.S. travelers might not be following CDC travelers' health recommendations for international travel. Public Health Action: GeoSentinel surveillance data have helped researchers define an evidence base for travel medicine that has informed travelers' health guidelines and the medical evaluation of ill international travelers. These data suggest that persons traveling internationally from the United States to developing countries remain at risk for illness. Health-care providers should help prepare travelers properly for safe travel and provide destination-specific medical evaluation of returning ill travelers. Training for health-care providers should focus on preventing and treating a variety of travel-related conditions, particularly traveler's diarrhea and malaria.