"Taking a 'nu' Look : Studying Low Energy Solar and Terrestrial Neutrinos with Borexino" Solar neutrino experiments have taught us a great deal about neutrinos, neutrino oscillations, and the nuclear fusion reactions that power the sun. The Borexino experiment, in which the Princeton particle astrophysics group plays a leading role, has made important new solar neutrino measurements by extending neutrino spectroscopy to very low energies. In this talk I will review what we have learned (and are still learning!) about solar neutrinos and the Sun. I will also describe our first measurement of the flux of terrestrially produced "geo-neutrinos" and how this and similar measurements can allow us to study the chemical composition of the Earth's interior and to better understand the planet's thermal history.