This is a free textbook for an undergraduate course on the Theory of Computation, which we have been teaching at Carleton University since 2002.Until the 2011/2012 academic year, this course was offered as a second-year course (COMP 2805) and was compulsory for all Computer Science students. Starting with the 2012/2013 academic year, the course has been downgraded to a third-year optional course (COMP 3803).