Sexually Transmitted Diseases (STD) are a major health problem all over the world. The diseases are often spread by unsuspecting asymptomatic individuals. One important means of controlling STD is thus the identification of asymptomatic persons. The purposes of this thesis were a) to describe methods of identifying infected individuals through contact tracing and screening, b) to evaluate contact tracing routines, c) to compare epidemiological characteristics of two different groups of chlamydia-infected women and their partners and d) to see if data from contact tracing could be used in a model describing partner choice and STD spread. A microepidemic of penicillinase-producing gonococci was effectively controlled through contact tracing by cooperating counsellors. Serotyping of gonococci and graphic description proved of great help in mapping the contact chain. Contact tracing integrated in the psychosocial long-term care of HIV-infected patients resulted in identification of a great number of previously unidentified individuals. Screening for Chlamydia trachomatis (Ct) among asymptomatic patients of family planning clinics revealed a 7.3% Ct prevalence. Four factors were significantly correlated to the risk of being infected: age 18-23, duration of present relationship less than 1 year, failure to use condoms and no previous history of genital infection. Abstention from testing was especially high in areas with low socio-economic status. In these areas, increased resources for health care are needed. Five different levels of management of Ct infections were compared. Increased measures to verify that reported partners were examined decreased reinfections and after Ct was included in the STD Act more reported partners than before came for examination. Index patients found by chlamydia screening and their partners have a lower average number of recent partners than index patients visiting an STD clinic and their partners. Partners of female patients in the STD clinic group were also more often Ct-positive. The differences between the groups are small and do not justify different ambitions in partner management. Choice of steady partner was rather restricted and choice of casual partner followed a more random pattern. Our data thus do not support a pure random mixing model for STD spread.