centre in Amsterdam, and the Forschungszentrum Jülich in Germany. Sensitivity The mission of LOFAR is to map the Universe at radio frequencies from ~10–240 MHz with greater resolution and greater sensitivity than previous surveys, such as the 7C and 8C surveys, and surveys by the Very Large Array (VLA) and Giant Meterwave Radio Telescope (GMRT). LOFAR will be the most sensitive radio observatory at its low observing frequencies until the Square Kilometre Array (SKA) comes online in the late 2020s. Even then, the SKA will only observe at frequencies >50 MHz and LOFAR's angular resolution will remain far superior. Science case The sensitivities and spatial resolutions attainable with LOFAR make possible several fundamental new studies of the Universe as well as facilitating unique practical investigations of the Earth's environment. In the following list the term is a dimensionless quantity indicating the redshift of the radio sources seen by LOFAR. In the very distant Universe (), LOFAR can search for the signature produced by the reionization of neutral hydrogen. This crucial phase change is predicted to occur at the epoch of the formation of the first stars and galaxies, marking the end of the so-called "dark ages". The redshift at which reionization is thought to occur will shift the 21 cm line of neutral hydrogen at 1420.40575 MHz into the LOFAR observing window. The frequency observed today is lower by a factor of 1/(z+1). In the distant "formative" Universe (), LOFAR is capable of detecting the most distant massive galaxies and will study the processes by which the earliest structures in the Universe (galaxies, clusters and active nuclei) form and probe the intergalactic gas. In the magnetic Universe, LOFAR is mapping the distribution of cosmic rays and global magnetic fields in our own and nearby galaxies, in galaxy clusters and in the intergalactic medium. The high-energy Universe, LOFAR detects the ultra high energy cosmic rays as they pierce the Earth's atmosphere. A dedicated test station for this purpose, LOPES, has been in operation since 2003. Within the Milky Way galaxy, LOFAR has detected many new pulsars within a few kpc from the Sun, has searched for short-lived transient events produced by stellar mergers or black hole accretion, and will search for bursts from Jupiter-like extrasolar planets. Within the Solar System, LOFAR detects coronal mass ejections from the Sun and provide continuous large-scale maps of the solar wind. This crucial information about