The first half of the book focuses on the deductive engine that underlies this application, the foundations that users do not see. After giving a mathematical model of traditional spreadsheet applications, we extend them with operators to perform a number of relational tasks, similar to the user view of a database but in a spreadsheet context. Expressing this extension in a logic programming framework is a natural step towards giving it powerful deductive capabilities. The second half of the book deals with the user interface, the part of the application with which the user actually interacts. We review the elements of the graphical user interface of traditional spreadsheet applications and describe practical methodologies for designing user interfaces borrowed from the field of cognitive psychology. We then propose a design that conservatively integrates mechanisms for a user to take advantage of the new deductive capabilities. This is followed by the results of some preliminary usability experiments.