Introduction to Logic has been the gold standard in introductory logic texts since its first edition in 1953. The 15th Edition of Introduction to Logic improves on its many strengths and it introduces new and helpful material that will greatly assist both students and instructors. In particular, instructors will find that Chapters 1, 8 and 9 have been greatly enhanced without disturbing the book's clear and gradual pedagogical approach. Chapter 1 now uses a simpler and better definition of "deductive validity" that improves the rest of the book, and especially chapters 1, and 8-10. Chapter 9 now has simpler definition of "simple statement" and "compound statement," more examples of the Complete Truth-Table Method, a detailed, step-by-step account of the Shorter Truth-Table Method (with detailed step-by-step examples for conclusions of different types), a more complete and detailed account of and justification for Indirect Proof, and a new section on Conditional Proof, which complements the 19 Rules of Inference and Indirect Proof. For the first time, Introduction to Logic also explicates proofs of tautologies using both Indirect Proof and Conditional Proof. Additionally, a new section at the end of Chapter 9 explains the important difference between sound and demonstrative arguments, thereby clarifying valid arguments with inconsistent premises and valid arguments with tautological conclusions.