The Bridging Professional Development project (which we refer to as Bridging PD) helps middle school mathematics teachers learn to teach for argumentation. Mathematical argumentation is a fundamental practice of mathematicians. We treat it as a social process in which students make conjectures and justify mathematical statements in classroom discussion. The purpose of an argument is to find out as a group whether a mathematical statement is true or not. The Common Core Standards for Mathematics (Common Core State Standards Initiative, 2010) have given new urgency to the need for argumentation to be taught in every classroom. To “construct viable arguments and critique the reasoning of others” is one of eight practice standards in the Common Core, along with others such as problem-solving and reasoning abstractly. The practice standards are relatively new to many teachers and are an important lever for engaging students in conceptually rich mathematical discourse.