Abstract: Neutrinos created in the sun oscillate between flavors as they travel to Earth. This phenomenon, which has recently been confirmed by the SNO and Kamland experiments, has not yet been measured by a direct counting experiment for the main flux of low energy neutrinos from the sun. Borexino will first make this measurement and thereby provide a very sensitive test of oscillation physics, and of our understanding of solar neutrino physics. I will describe the key main technical challenge of the experiment reducing the radioactive background some 10 orders of magnitude from ambient levels. In part this is accomplished by locating the experiment deep underground, in a site in Gran Sasso Italy. Data taking should begin in 2004. I will conclude with brief description of the prospects for another exciting astrophysics measurement which must be made underground: detection of WIMP dark matter.