Lake Päijänne LTER site constitutes one of the biggest lakes in Finland (and smaller lakes in catchment area) with almost 300 000 habitants in surrounding areas. The lake has been heavily polluted but its condition has been improved, and today the lake has both high economic and social importance for a significant proportion of the Finnish population. There is a long experience of lake research in the L. Päijänne area, including some long-term data series. Research at L. Päijänne is diverse and is already well-equipped with instrumentation supporting long-term monitoring. The research covers large scales from hydrology to fish stock assessment. This research area includes also another large Finnish lake, Lake Konnevesi, as a natural reference lake in the northern branch of the Lake Päijänne system.