This is the first book to offer a complete account of the theory of simplicial decompositions of graphs, possibly the single most important tool in infinite graph theory. The text is centered on a number of guiding problems and concepts such as the existence and uniqueness problem of simplicial decompositions into primes, and the concept of excluded minors as a means of identifying a desired structure. Following theoretical developments since the 1930s, the author includes extensive information on the current state of research, as well as many examples, proof strategies, exercises, and ongoing problems. The book will be a useful resource for graph theorists and mathematicians, and its lively account of important research will interest advanced students as well.