This thesis investigates how variations in the oxygen isotopic composition of lake waters in northern Fennoscandia are recorded in lake sediment archives, especially diatoms, and how these variations can be used to infer past changes in climate and atmospheric circulation. Results from analyses of the oxygen isotopic composition of lake water samples (δ¹⁸O_lakew) collected between 2001 and 2006 show that δ¹⁸O of northern Fennoscandian lakes is mainly controlled by the isotopic composition of the precipitation (δ¹⁸O_p). Changes in local δ¹⁸O_p depend on variations in ambient air temperature and changes in atmospheric circulation that lead to changes in moisture source, vapour transport efficiency, or winter to summer precipitation distribution. This study demonstrates that the amount of isotopic variation in lake water δ¹⁸O is determined by a combination of the original δ¹⁸O_lakew, the amount and timing of the snowmelt, the amount of seasonally specific precipitation and groundwater, any evaporation effects, and lake water residence time. The fact that the same isotope shifts have been detected in various δ¹⁸O_lakew proxies, derived from hydrologically different lakes, suggests that these records reflect regional atmospheric circulation changes. The results indicate that diatom biogenic silica isotope (δ¹⁸O_diatom) records can provide important information about changes in atmospheric circulation that can help explain temperature and precipitation changes during the Holocene. The reconstructed long-term Holocene decreasing δ¹⁸O_p trend was likely forced by a shift from strong zonal westerly airflow (relatively high δ¹⁸O_p) in the early Holocene to a more meridional flow pattern (relatively low δ¹⁸O_p). The large δ¹⁸O_lakew depletion recorded in the δ¹⁸O records around ca. 500 cal yr BP (AD 1450) may be due to a shift to more intense meridional airflow over northern Fennoscandia resulting in an increasing proportion of winter precipitation from the north or southeast. This climate shift probably marks the onset of the so-called Little Ice Age in this region.