Chapter 15. Theory of Combinatorial Games Aim: To present a systematic development of the theory of combinatorial games from the ground up. Approach: Computational complexity. Combinatorial games are completely determined; the questions of interest are efficiencies of strategies. Methodology: Divide and conquer. Ascend from Nim to Chess and Go in small strides at a gradient that is not too steep. Presentation: Mostly informal; examples of combinatorial games sampled from various strategic viewing points along scenic mountain trails illustrate the theory. Add on: Atasteof constraint logic a new tool to prove intractabilities of games.