The concept of truth has been central in all students' mathematical experiences across all grades, and the success of instilling the concept highly depends on the mathematics teachers' knowledge of proof. The study in this article examines knowledge of proof by mathematical induction among elementary and secondary mathematics teachers and is based on interviews of 95 participating teachers. Finding reveal that teachers from both groups had difficulties around the essence of the basic step of the induction method, the meaning associated with the induction step, and the possibility that the truth set of a sentence in a statement proved by mathematical induction to include values outside its domain of discourse.