What do road and railway systems, electrical circuits, mingling at parties, mazes, family trees, and the internet all have in common? All are networks - either people or places or things that relate and connect to one another. Only relatively recently have mathematicians begun to explore such networks and connections, and their importance has taken everyone by surprise. The mathematics of networks form the basis of many fascinating puzzles and problems, from tic-tac-toe and circular sudoku to the 'Chinese Postman Problem' (can he deliver all his letters without traversing the same street twice?). Peter Higgins shows how such puzzles as well as many real-world phenomena are underpinned by the same deep mathematical structure. Understanding mathematical networks can give us remarkable new insights into them all.