Since its publication in 1989, David Sanford's If P Then Q has become one of the most widely respected works in the field of conditionals. This new edition includes three new chapters, thus updating the book to take into account developments in the area over the past fifteen years. Part One gives an historical overview of the history of philosophical treatments of conditionals, from ancient times to the contemporary development of possible worlds. In Part Two, Sanford puts forward his own treatment of conditionals.