In the last few years, important discoveries have been made in the field of low energy neutrino physics. Flavour transitions of solar electron to muon and tau neutrinos have been clearly shown in the SNO experiment. Further evidence for neutrino oscillations has been found in the KamLAND experiment by a disappearence measurement of electron antineutrinos emitted by nuclear reactors. In 2007, the Borexino experiment started to take data, demonstrating the great potential of the liquid-scintillator technique for the detection of solar and geoneutrinos. Based on this experience, LENA has been proposed as a next-generation neutrino detector: Based on a 50 kiloton target of liquid scintillator, LENA will use low-energy neutrinos as probes for studying astrophysical objects like the Sun, core-collapse Supernovae and the Earth itself. The scientific program will also cover elementary particle and neutrino physics as well as the search for nucleon decay.