Part I:  The Landscape of Discrete Mathematics in the School Curriculum -- Discrete Mathematics is Essential Mathematics in a 21st Century School Curriculum (and introduction to this volume) -- The Absence of Discrete Mathematics from Primary and Secondary Education in the United States  and Why that is Counterproductive -- Discrete Mathematics in Lower School Grades? Situation and Possibilities in Italy -- Discrete Mathematics and the Affective Dimension of Mathematical Learning and Engagement -- Part II: Combinatorics and Combinatorial Reasoning -- Combinatorial Reasoning to Solve Problems -- Children's Combinatorial Counting Strategies and their Relationship to Mathematical Counting Principles -- Reinforcing Mathematical Concepts and Developing Mathematical Practices through Combinatorial Activity -- Complex Mathematics Education in the 21st Century: Improving Combinatorial Thinking based on Tamás Varga's Heritage and Recent Research Results -- Part III: Recursion and Recursive Thinking -- Discrete Dynamical Systems: A Pathway for Students to Become Enchanted with Mathematics -- How Recursion Supports Algebraic Understanding -- Part IV: Networks and Graphs -- Food Webs, Competition Graphs, and a 60-year-old Unsolved Problem -- Graph Theory in Primary, Middle and High School -- Part V: Fair Decision-Making and Game Theory -- Fairness -- Mathematical Research in the Classroom via Combinatorial Games -- Machines designed to play Nim games (1940-1970): A possible (re)use in the modern French mathematics curriculum? -- Part VI: Logic and Proof -- Mathematics and Logic: Their Relationship in the Teaching of Mathematics.