To advance our understanding of the formation, evolution and structure of extra-solar planetary systems we are building a high- precision imaging polarimeter (ExPo). ExPo will initially be located at the 4.2m William Herschel Telescope on La Palma. We will use polarimetric techniques similar to those developed for high-precision solar polarimetry to reach a sensitivity of 10-̂5, to polarimetrically image and characterize planets and protoplanetary debris discs. I will present a review of the proposed data analysis techniques and science goals that will be achievable using the significant improvement in polarimetric imaging capabilities.