Nicholas Jardine offers here an edition and the first translation into English of Johannes Kepler's A Defence of Tycho against Ursus. He accompanies this with essays on the provenance of the treatise - the circumstances which provoked Kepler to write it, an analysis of its strategy, style and historical sources and of the contents of Ursus' Treatise on Astronomical Hypotheses to which Kepler was replying. Dr Jardine also provides three extended interpretive essays on the intrinsic interest and historical significance of the work.