This research investigated pre-service secondary teachers understanding of the term viable argument a new term introduced with the Common Core Standards. The research investigated how they define and understand viable argument, how they construct arguments, and how they evaluate mathematical arguments. The research was conducted using semi-structured interviews with five pre-service teachers that have completed the majority of their mathematical coursework. The results showed that the pre-service teachers compared viable arguments to mathematical proof and they recognized both similarities and differences between these terms. The participants were found to have the understanding of proof, reasoning, and argumentation that will be needed to implement the recommendations of the Common Core Standards. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]