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Arboviruses transmitted by mosquitoes and ticks represent an increasing public health concern due to expanding vector distributions and environmental change. In Sweden, tick-borne encephalitis virus (TBEV) and Sindbis virus (SINV) are endemic arboviruses, although the true extent of human exposure remains incompletely characterised. This thesis evaluated multiplex serological approaches for arbovirus surveillance through application of an established TBEV suspension multiplex immunoassay (SMIA) and development of a SINV suspension immunoassay (SIA). A TBEV NS1/whole virus SMIA was applied to 1,614 anonymized serum and plasma samples collected in Sweden during 2024–2025. Marked regional differences in TBEV seroprevalence were observed, with the highest infection associated seroprevalence detected in Uppsala and Stockholm. For SINV assay development, multiple recombinant glycoproteins were evaluated, of which an insect cell-expressed SINV E1 glycoprotein demonstrated the strongest separation between positive and negative sera, combined with low background reactivity and good reproducibility. Application of the finalized assay to 914 Swedish blood donor sera identified substantially higher SINV seroreactivity in Sweden than previously reported, with apparent seroprevalence approaching 10% in Umeå and Gävle. Overall, the findings demonstrate the utility of multiplex serological approaches for large-scale arbovirus surveillance and indicate that SINV exposure in Sweden may be considerably more widespread than previously recognised.