The main results are that for the classical Sudoku the number of filled grids is 6,670,903,752,021,072,936,960 (), which reduces to 5,472,730,538 essentially different groups under the validity preserving transformations. There are 26 types of symmetry, but they can only be found in about 0.005% of all filled grids. A puzzle with a unique solution must have at least 17 clues, and there is a solvable puzzle with at most 21 clues for every solved grid. The largest minimal puzzle found so far has 40 clues.