This paper introduces S.P.L.O.T., a Web-based reasoning and configuration system for Software Product Lines (SPLs). The system benefits from mature logic-based reasoning techniques such as SAT solvers and binary decision diagrams to provide efficient reasoning and interactive configuration services to SPL researchers and practitioners. In addition, the system provides a feature model repository containing real and generated models to encourage knowledge sharing among researchers in the field. Categories and Subject Descriptors D.2.2 [Software Engineering]: Design Tools and Techniques—CASE; D.2.1 [Software Engineering]: Requirements/Specifications—Tools General Terms Design Keywords Software Product Lines, Feature Models, Automated Reasoning, Interactive Configuration, Feature Model Analysis