The purpose of the third edition of this book is to give a sound and self-contained (in the sense that the necessary probability theory is included) introduction to classical or mainstream statistical theory. The book is intended to be a textbook, aimed for use in the traditional full year upper-division undergraduate course in probability and statistics or for use as a text in a course designed for first year graduate students. No previous course in probability or statistics is needed in order to study the book. The book is designed to be used in either the quarter system or the semester system.