TITLE Probing Inflation with SPIDER, a Balloon-Borne CMB Polarimeter ABSTRACT The generation of a stochastic gravitational wave background is a key prediction of cosmological theories of inflation. At large angular scales, these gravitational waves would imprint a "B-mode" polarization pattern in the Cosmic Microwave Background, providing a new window into the physics of the early universe. SPIDER is a balloon-borne telescope that has been uniquely optimized to search for the inflationary B-mode signature in the CMB. Over the course of two Antarctic flights, SPIDER will make polarization maps over 10% of the sky in three frequency bands with degree-scale angular resolution. After an overview of the instrument and science goals, preliminary results from SPIDER’s 2015 flight will be presented along with a summary of progress towards the second flight.