This book examines the five key elements (meaningful use of symbols, mindful manipulation, reasoned solving, connection algebra with geometry, and linking expressions and functions) identified in "Focus in High School Mathematics: Reasoning and Sense Making" in more detail and elaborates on the associated reasoning habits. This volume is one of a series of books that support National Council of Teachers of Mathematics' "Focus in High School Mathematics: Reasoning and Sense Making" by providing additional guidance for making reasoning and sense making part of the mathematics experiences of all high school students every day. This book includes three chapters: (1) Algebra and Geometry; (2) Building Equations and Functions; and (3) Formal Algebra. Preface, Epilogue and References are also included. Appended are: (1) NCTM Standards and Expectations for Grades 9-12; (2) Algebra Standard; (3) Number and Operations Standard; and (4) Measurement Standard.