When using all its advantages of clinical vigilance, direct communication,and fast feedback, a clinical surveillance network can be remarkably effective in detecting sentinel events and in translating the new information into modifications of clinical practice. Travelers have great advantage when serving as surveillance tools for imported diseases. They travel widely and potentially expose themselves to all types of infectious diseases, they are very mobile, and they return during the incubation period of most diseases to a medical system that is capable of achieving fast and definitive diagnosis. Clustering of infections in returnees can be used immediately to warn outbound travelers of a particular risk and to increase their protection. In addition, travelers can also serve as "canary birds" for disease outbreaks in developing countries that might not be able to provide facilities for fast diagnosis. Information derived from returning travelers can be invaluable for the host country if channeled back to the medical authorities. TropNetEurop screening for increases in unexpected notifications has proved to be a sensitive early warning tool for the detection of increased transmission rates in endemic countries. For the future, it is hoped that traditional surveillance systems and recently introduced networks will be able to cooperate more fully. All systems have strengths and weaknesses and can gain from information provided by each other. Linkage of existing networks, which avoids duplication of work and fully exploits the information potential of all combined systems, should be targeted.