CLICK ON 'COSTELLO IRELAND' ABOVE FOR MORE GREAT PUZZLE BOOKS! Sudoku is a logical number placement puzzle. The goal is to fill a 6 x 6 grid with digits so that each column, each row and each 2 x 3 sub-grid contain all of the digits from 1 to 6, used only once in each column, row and sub-grid. A well-constructed Sudoku puzzle has only one solution.