The Square Kilometre Array (SKA) radio telescope together with the E-ELT ( and other extremely large optical telescopes) will be one of the dominant new ground based astronomy instruments of the first half of the 21st century. The SKA will cover the frequency range 50MHz to 20GHz (6m to 1.5cm wavelength) and will be built on sites in Australia (mostly low frequencies) and South Africa (high frequencies). The capabilities of the SKA in terms of sensitivity and sky survey speed will be orders of magnitude greater than existing radio instruments. The SKA will tackle both a series of key questions and also be a general user instrument for adding radio observations to multi-wave and studies of all kinds. In this general support role the SKA will be complementary to other Swedish supported southern hemisphere instruments (ESO optical/IR telescopes and ALMA). Amongst its key projects SKA will seek to image neutral hydrogen (HI) in the Epoch of Re-ionisation, probing the era of galaxy formation. The SKA will also trace the evolution of the galaxy population over cosmic time; using HI to follow the build-up of dynamical and atomic gas mass and using radio continuum to trace star-formation and AGN activity. Via measuring the angular scales of correlations in the galaxy distribution imprinted by Baryonic Acoustic Oscillations and via observations of gravitational lensing the SKA will seek to constrain the equation of state of Dark Energy. The SKA will observe Pulsars as unique probes of extreme physical conditions testing the properties of nuclear matter and testing General Relativity in the strong field limit. By accurate timing of pulsars over the sky the background of low frequency gravitational waves from merging compact objects or from the early universe can be detected. As a unique probe of magnetic fields radio observations with the SKA will be used to trace the origin and evolution of cosmic magnetism. The SKA will also make vital contributions to studies of star and planet formation and answering the questions of cosmic origins. Large pre-biotic molecules can be observed in the ISM and, at the highest frequencies, proto-planetary disks can be observed.Finally the SKA will be the premier instrument for the radio detection of artificial intelligent signals either serendipitously or as part of targeted searches. As well as presenting the main science goals the present status of the SKA project will be described. Unlike most existing radio telescopes access to the SKA will only be via membership of the international consortium that builds the SKA - therefore continued access to state-of-the-art observing facilities in the centimetre band for Swedish astronomers in the decades to come requires Swedish participation in the project.
