Improving your partnership's ability to overcome interference is essential for good results. Finding the right contract requires a clear understanding of the values represented by responder's bids after the opponents interfere. Duplicate Bridge 403: Defending Second Seat Interference provides the reader with two sets of tools for this: 1.Hand valuation in competition using an algorithm developed by Lawrence and Wirgren. This method of hand evaluation provides the reader with a competitive advantage relative to older methods of estimating trick count. 2.A comprehensive set of bidding structures is provided for defending against bids made in the second seat. Where possible, alternatives are detailed giving partnerships a choice that fits their individual style. Quizzes follow each section allowing the reader to test their understanding. Discussions include defending against: -A Takeout double -Natural overcalls using Negative Doubles -Two-suited overcalls such as Unusual 2NT, Michaels, and Roman Jump Overcall -Two-level opening preempts using the Lebensohl relay -Natural overcalls of a 1NT opening (Lebensohl relay) -1NT overcalls with one known suit and one unknown suit (Lebensohl relay) -1NT overcalls with two known suits (Lebensohl relay) Bidding structures for Lebensohl were based on "The Lebensohl Convention Complete" by Ron Andersen. This text is designed to improve your partnership understandings in competition. Given the frequency opponents interfere in today's game a small improvement and can quickly lead you to stronger results.