Sexually transmitted infections (STIs), including HIV, are major global health concerns. Mozambique ranks eighth in the world for HIV prevalence (12.6% in the adult population). Proper diagnosis and treatment are important for prevention and control of the spread of STI/HIV. Untreated STIs are associated with numerous complications, and increased transmission and acquisition of HIV. This thesis aims to evaluate and improve methods of diagnosing STIs, including HIV, in Mozambique. In study I, the impact of vaccine-induced seroreactivity (VISR) on the accuracy of HIV diagnostic algorithms in Tanzania and Mozambique was assessed using stored serum/plasma samples from previous HIV vaccine trials. A substantial part of the samples was misclassified as HIV-infected using standard HIV diagnostic strategies based on antibody detection. In study II, the accuracy of a 4th generation rapid diagnostic test (RDT) in detecting acute and seroconverted HIV infection was evaluated in samples collected from sexually active women in Maputo, Mozambique, and in three commercial HIV-1 seroconversion panels. The antibody component of the 4th generation RDT performed comparably to the Mozambican 3rd generation HIV algorithm while the antigen component showed low sensitivity in detecting acute infection. In study III, vaginal/cervical samples collected from a cohort of sexually active women with urogenital complaints in Maputo were used to assess the accuracy of vaginal discharge syndromes in diagnosing four non-viral STIs. High prevalences of chlamydia, trichomoniasis and HIV-1/2 were recorded. Syndromic management of vaginal discharge revealed low specificity in detecting four non-viral STIs leading to misclassification and overtreatment of cases. In study IV, we determined the prevalence of bacterial vaginosis (BV) using a BV multiplex real-time PCR assay among women in Maputo. A very high prevalence of BV associated with certain sociodemographic and behavioural factors was recorded.