Puumala viruses, a member of the Hantavirus genus in the Bunyaviridae family, are enveloped by a lipid bilayer and possesses a tripartite single stranded RNA genome with negative polarity. The hantaviruses encode four proteins: a nucleocapsid protein (N), two membrane spanning glycoproteins (GN and GC) and a RNA dependent RNA polymerase (RdRp). Hantaviruses cause two forms of diseases, hemorrhagic fever with renal syndrome (HFRS) in Europe and Asia, and hantavirus pulmonary syndrome (HPS) in the Americas. The hantaviruses are mainly rodent borne, and humans are mostly infected by inhalation of aerosolized rodent secrete. Human Puumala virus infection results in nephropathia epidemica (NE), a mild haemorrhagic disease. It is of importance to have a good understanding of the epidemiology and genetics of these viruses for the development of new diagnostic methods and for future vaccine development. In this thesis we determined the complete viral genome sequence and characterized the structural proteins based on studies of expression and glycosylation patterns, for a unique human virus isolate; performed a genomic analysis of local Puumala viruses and their individual rodent host, Clethrionomys glareolus, from six different locations was performed. It was seen that the virus genetic variation between different locations could be stable over relatively large distances while there could be large variation over a short distance. For the bank voles no such variation could be seen; developed and evaluated Genetic vaccines, based on PCR-generated linear DNA. We showed that it was important to protect these fragments against nuclease degradation at that attachment of a nuclear localization signal peptide further improved the immune response. We also designed, fabricated and evaluated a 2000 probe cDNA-microarray for identification and differentiation of hantaviruses. The chips was based on 12 different strains of six hantaviruses and could differentiate between both different hantaviruses and strains within one hantavirus serotype.